I want to start off by saying yes, my game is modded. I've been troubleshooting and updating for almost 4 hours and I'm exhausted, so I'm posting here in case someone is having the same issue and/or has solved it. UPDATE! I took out all my mods and repaired the game, and the issue persists. So, not a mod issue.
When I load into a game, old save or new, sims that are related are not actually related in-game. It doesn't show a family relationship in the relationship panels. When I enter CAS it's all there, and once I exit out of CAS (by applying, not by clicking the x) they suddenly ARE related, like they should have been from the start. Because of this, sims will get whims/wants where they want to do romantic things with family members, and the romance social menu is also unlocked.
I'll look into it again tomorrow but I'm way too tired right now, hoping someone has had a similar experience and solved it.
As it turns out the hotfix actually didn't fix anything so I shall wait 🥴
My character married another character, they had several children. Then one day I looked in CAS, and noticed that 2 of the kids were listed as "household member" instead of their own kids. I figured it might be a MCCC error, so next time she got pregnant I took MCCC out before the baby was born.