Mr.Paul1967, I think Mr.Atheist said it quite clearly and I happen to agree with him. If back then a tribal custom constitutes the law of the tribe then there's nothing ethical in what the head of the family want to impose upon his household.

What could be unethical in the story was the woman's scheme of deception.
Did she really wanted to have a child even if it meant getting pregnant by her father-in-law?
Or was she just kinda lonely and horny and just wanted to get laid?
. . . so her twins would both be her sons and also her brothers-in-law, right?

Is there a clear passage in whatever holy book that the almighty creator commands the brother-in-law to have the widow of his brother pregnant by him, or, be the husband of her sister-in-law? What if the guy already have a wife and kids? Those are very convenient excuses to circumvent the law of any land.Well, it's been happening ever since, we weren't born yesterday.

And why did God kill the sons of His servant (Judah) because his sons were evil? Why not just give the burden to Judah to discipline and educate his sons? if Judah fails then he should be the one killed first . . . and there could even be a small chance that his sons will change their evil ways out of fear