My dating sims are now sisters

I have two sims that were/are dating, they have one kid together and now when I open the game every time I go into CAS, (and even when I don’t open CAS it eventually happens), CAS just always causes it no matter what,) they become sisters and can’t preform romantic interactions aside from a massage. I played without cc or mods and this is still a problem. Idk how to fix it and using cheats or mods such as MCCC doesn’t work. I’ve tried moving them in separate households and everything but even in separate households once the one sim goes into CAS it sets them as sisters once again. I have noticed it’s only when that once sim goes into CAS though. The other one can do that same thing and they don’t become sisters. But now the game save is ruined and their son is the non-broken sims nephew and it’s really annoying. I’ve seen people complaining of this happening as long as 7 years ago but I just had this happen after the last big update that broke pregnancy and everything. (I’m on a windows PC)