Partnered Sims turn into siblings

It's what the title says, random sims are incorrectly assigned as siblings.

My male sim had a child with his partner, and their relationship was completely normal, including a high romance bar, but then the game suddenly marked her as his sister in the family tree.

I tried to fix it by saving her to the gallery, deleting her entirely from the game, restarting the game, and placing her back in. After that, I had to restart from scratch by making them meet again and rebuilding their relationship. Despite all of that, the same thing happened again, and she was once again added as his sister.

I thought this bug only occurs to those in romantic relationships, but now I’m seeing it affect other sims in my played households as well. Sims I control are being assigned random siblings they are not actually related to and have never met. The game is constantly pulling other sims into the family tree incorrectly. The only way I’ve been able to deal with it is by deleting those sims, which shouldn’t be necessary.

This is seriously damaging my save. Relationships are getting rewritten randomly, and it’s making my generational gameplay impossible to continue.