My family tree keeps breaking

I've been dealing with this issue for months and I'm just finally at my breaking point with it. I am on the verge of tears (silly, I know, but... ugh, I just can't take it anymore).

My Sim is married to Alexander Goth. They have 3 children together; one adult daughter who doesn't live with them anymore and 2 teens (a pair of boy/girl twins). They also have 4 grandchildren through their adult daughter.

For whatever reason, my family tree keeps breaking every time I go into CAS. The family tree shows my Sim and her husband are still married but somehow, also siblings. They are definitely not siblings; I don't play that kind of stuff. They've never been step-siblings or adopted siblings, either. They are not related through blood or adoption at all but the game keeps thinking they are both siblings and spouses.

This messes up a lot of the rest of the tree, too. The family tree will incorrectly assign either Bella or Mortimer or both as my Sim's parents and Alexander's sisters as my Sim's sisters, too.

Both my Sim and Alexander keep forgetting they have an adult daughter, too. The teenage twins stay on the tree but their older sister constantly disappears from the tree.

I've used MCCC to completely undo ALL the family ties between everyone and reapply them with MCCC so many times but it never sticks for long. I've gone so far as to move everyone over to a new save and delete problematic Sims from the game and replace them with a clone. These problems still persist.

I do play with mods usually and I know the first course of action should be to test without mods but I'm not sure how to do that in this particular case; it's not possible to fix family ties to this extent without using mods (at least MCCC).

Is there any way to fix this problem for good? I've been playing this save for 5+ years and it would crush me to never be able to fix it (most of my Sims are immortal and/or occults so I only have 4 generations; it's not one of those 50+ generation legacies).