Don't defend you 'lack of faith' in religion, point out that extraordinary claims require extraordinary evidence of which histroy falls short then focus on your abundance of faith in reason, logic and science.
Also Ricky Gervais made a great point 'If all religious writings were to disappear right now, in a 100 years their replacements will be totally different. If all scientific facts and documentation were to disappear then in 100 years all the same facts and theories would reappear because they're based on evidence and are in fact 'real'. He also said at the end of the golden globes hosting 'I want to thank god for making me an atheist'.
Most history is written by the victors and is partial at best.

There is no proof of god or even that there ever was a person called Jesus until 81 years after his purported death they are relying solely on faith which is nothing more than self delusion.
My suggestions that you tell them point blank to respect your lack of self delusion and you will respect their delusions.

I say " It's a nice story, and I wish it were true, but it's not." There is no madman in the sky running this freakshow.

I don't, anymore. Logic doesn't work against deeply ingrained emotional indoctrination. Any time you threaten these deeply held beliefs, which gets heavily tied into their identity, in order to prevent the cognitive dissonance that would result from accepting your stance, their egos would double down to protect that from happening. Who would they be in the absence of God?

And, it's also built into the belief system what would happen to them if they turned against God. That brainwashing is really dangerous and painful to overcome.

I have heard a to of good arguments on "The Atheist Experience" show.
[atheist-experience.com]
They often use the words directly from the Bible to show how irrational it is to believe in it as a source of morality. I've read the Bible quite a bit as an adult, and that is what led me to non-belief. Most Christians I know only read certain stories, and never read the parts in between. On the show I linked to, Christians often argue that the verse the host quotes on the show doesn't mean what they say it does, even though they read it word for word.

You might try listening to a few episodes. I listen while driving, it makes my commute more bearable.

"...possess absolute truth, and be the word of God..."

Ask them: If the bible is the true word of god, what ever were the ecumenical councils for - especially the First Council of Nicaea? Why were they needed if god gave his words to all the bible's authors? You'd think that all of those authors and priests and brothers got the same message, right?

I don't I would never rip someones comfort blanket off them and I think its wholly personal whether you want a god or not - you seem to be doing very well - why would you need yours sisters to be at your side on this one they have their own lives to live -just like you.

Well, I'm in the camp that people who buy into religion in general become fairly heavily invested in not changing, so I would probably not try. If they want to pray for you, just ask them to pray that you're on the path that's right for you, and leave it at that.

It's worth getting to know the Bible so you can stand your ground. It is pretty easy to demonstrate as an unreliable witness.

But in the mean time, here is a dead simple one to start with: Slavery.

Is chattel slavery moral or immoral? As a species, we've come to the conclusion it is not moral. Why does God not only fail to condemn the practice, but actually sanctions it instead. Rules of slave ownership are provided!

Jesus was totally down with slavery, too. Luke 12:47 and Matthew 24:51 are two prime examples.

These are not words from Jesus's lips, but really make completely clear where the NT stands on slavery.

Ephesians 6:5-8 (NASB): Slaves, be obedient to those who are your masters according to the flesh, with fear and trembling, in the sincerity of your heart, as to Christ; not by way of eyeservice, as men-pleasers, but as slaves of Christ, doing the will of God from the heart. With good will render service, as to the Lord, and not to men, knowing that whatever good thing each one does, this he will receive back from the Lord, whether slave or free.

Just don't. Establish criteria for believing things in general. If they do not do the same, do not even engage -- whoever does not have criteria for believing or rejecting claims does not deserve a seat at the adults table. If they do, examine other religions according to these criteria first to see if they are good. And if only Christianity passes, hail us.

Each person has a way of seeing themselves in this world. Their perception is their reality. People raised in/or choose a faith-based religion, have a need that is fulfilled by that religion. To challenge and discredit their faith is to discredit them and will always be met with resistance.

I suggest you might wish to try to agree to respect their choice and ask for respect of your choice. When asked what your reasons are for not believing, answer honesty about why and how "you" came to that choice without criticising their belief. Who knows, you might plant a seed that inspires them to question and do some research.

They are your sisters, enjoy them. There is so much more to the relationship than religion.

You can point to all the parts in the bible that show a complete lack of basic scientific knowledge (like that stars can fall on Earth or that the moon is a source of light). An all-powerful, timeless god knows less than a modern 6 year old? Just keep pulling away at the threads until they realize their belief is founded by nothing.

We can not convert everybody. The fact that they know the Bible tells mw they are not going to be easy to covert.

I would Jesus Interrupted by Bart Erhmsn. I think that will help greatly. He discusses the origins of Christianity.

Pick out some of the really crappy parts and ask them if they support slavery, stoning, etc.

Or the just silly parts like not eating pork or shellfish.

I think we would need to hear a specific claim before being helpful. That being said, I'm not sure I think everything in the Bible is fiction. I figure a portion of it is true. I think a good portion of historians think there is a good chance Jesus was a real person. But I'm not expert.
Here is a pretty good discussion on the historic Jesus. [theskepticsguide.org]
It's not comprehensive, but still good.

I'm not sure I get your situation completely, but it seems important to you for you to feel like you are on solid logical grounds with your sisters.
I don't know your sisters, but that is very hard to do with most true believer. Weather it's a region or UFOs or whatever. They often see themselves as defenders of the light and they just double down on anything that goes against what they believe.
I think you have to find out what convinces yourself. If there are still unanswered questions about faith, I would explore those questions honestly. You should use your own judgement, and not the judgement of someone else's, but still try to remain open to changing your mind.

Take it easy, they are your family after all. But I think a good argument is, and Ricky Gervais spelt it out pretty well was the "story" of Noah. How the hell did he fit (best guess) 5 million pairs of species (so 10 million creatures) on to a boat and survive 150 days without any of them eating, killing, poisoning, stinging each other. That's also a lot of life to keep hydrated when stuck on a boat. And.. If the boat was actually that big to fit everything in it, where is evidence of said boat/Arc. And.. If it was just Noah, his wife and kids then we are all related, because they wouldn't be able to repopulate without incest.
There are a lot of good arguments out there, ask why dinosaurs aren't in the bible, that's was the first question I ever asked when I was a kid. There's plenty on YouTube. Richard Dawkins (author of The God Delusion) and Sam Harris put up awesome arguments.

You do not have to defend logic... They Have to defend Faith... Read The Collected works of Josephus. Read the Wars of the Jews, especially that part... You'll Find a High priest Named Jesus, and His father Joseph also a higher ranking priest getting slaughtered by their own people, around 65 AD or so, can't quite remember... Learn the customs of the Jews in that era 100 ± AD/BC. You will find all the ammo you need to help you keep your Faith in logic and if they don't get the Big pic after you tell them(If you do tell them) the real story behind the myth then well they are not as intelligent or informed as you thought they would have been, they have a mental retardation they can not overcome....Their supposed "Faith" would be better served in serving Mankind... OH and Forgot to mention a little Marcus Aurelius as well: Action is the only Truth... Adding, their faith should not reduce your love for them and as well, let them continue with respect and ask them to do the same for you...

Any more, I just shrug my shoulders and not care. The absurdity of their beliefs is crystal clear to me. If they can't see how irrational it is on their own, I don't have time to argue the obvious. I'll openly ridicule their beliefs if they try to convert me, though.

You don't. People are seldom persuaded by intelligent argument and evidence. It takes time, and regettably, a lot of knowledge about the suffering of human beings on this planet before people realize that any god out there which is all knowing and all powerful does not love us.

Once someone has you defending yourself on something like that, you've already lost. A mutually respectful discussion is fine, but as soon as you are feeling defensive about a logical position, or if someone else is acting defensive about an illogical position, it's time to hang it up. There is no way to defend logic from illogical people who are on the attack, and if a person feels defensive, they are not going to be open to new information even if you have clear evidence right in front of both of you.

anybody feeling defensive = debate over, at least until defensiveness is removed

I don't need to defend my non belief.

I recently read Leviticus. It amazed me how blood thirsty the Jewish god YAHWEH was. I doubt any modern person would want a god like that.... but there he is demanding animal sacrifices and the "sweet savor of burning blood". I suppose true believers will always come up with excuses for this kind of horrific behavior, But perhaps they should be challenged. It can't be easy

I don't think you need to defend your lack of belief. I have said often, the fact that you have to "believe" something means you don't have the evidence to back it up." once there is evidence, belief is no longer necessary. Beliefs are like apps. I've simply chosen not to install this particular app since it doesn't seem to help much with the rest of the system.