Leave them be, to each his/her own.

Not, so far. I hope your mom and sister snap out it soon. Critical thinking is the cure, but it sounds as though they be inoculated from that already.

I had an in-law that was convinced, and proud of it, that there were owls living around his SoCal residence. One afternoon, we were all having dinner outside when the "owl" made its signature sound. My in-law sat up and exclaimed that as evidence of owls. I knew right away that it was no owl, so I gently explained his error, but he wasn't buying it. So, an idea came to me. I quietly whipped out my smartphone, downloaded an mp3 file of the "owl", and played it for all to hear. He sat up again and exclaimed it as continued proof and that is when I held out my phone and played it again. It's a Mourning Dove, Jack. A Mourning Dove.

He had no choice but to concede and shut up after that. Ya, it was kinda cruel of me, but fuck his pride and ignorance.

You are right. There isn't much of a path forward unless you can get them reading something else. I went through that decline with my first wife. You become more of the enemy the more you try to help. If you can get someone outside of the family to intercede it might be easier and not focus things back towards yourself.