Please use this thread going forward: Partnered Sims turn into siblings | EA Forums - 4943482
First time posting in the forums so, apologies in advance.
In doing a little experimenting, the best I can figure out is that the game is trying to forcibly resolve "dotted-line" (step-children/step-parent) genealogical relationships when you enter CAS.
Very simple family tree:
Parent A (a Vampire) and Parent B (a Spellcaster) get married. Parent A has (non-occult) Son A from a previous marriage and brings him in to the household, making him a stepson to Parent B.
Parent A and Parent B have a child together, Daughter B.
Entering CAS at this point causes Parent A and Parent B to stay married, but in an attempt to justify the dotted-line relationship, Step-Parent B becomes "Uncle B" to Son A.
The game seems to have issues with the Stepchild/Stepparent relationship; I can't seem to encounter this issue with full blown children. Further, canceling out of CAS (instead of OK'ing your changes) seems to avoid the issue all together, which is why I'm thinking it's a bug with some sort of relationship reconciliation specifically when going into CAS.
I should note that I forcibly resolved the dotted-line relationship by making Son A a full-blown child of Parents A and B (making the son and daughter full siblings instead of stepsiblings). That also did not seem to fix the problem.
I hope this is helpful to someone working on this issue at EA. Sorry if this is...incomprehensible. I'm trying, I really am!
Did a little further testing with a save prior to my Sim learning any magic but still having the spellcaster occult trait. I can confirm now that the trait doesn't matter. This issue doesn't crop up unless you enter CAS after your Sim is duplicated either by the spell Duplicato or the science career's cloning machine, but I wanted to confirm now that I'm able to reliably reproduce this issue only after learning and using the Duplicato spell.
I cannot get it to reproduce prior to that point.
My working theory is that, because the cloned Sim is always listed as a sibling to the caster, some flag is getting set or some value's getting duplicated that's causing the "sibling" flag to get reassigned to the casting Sim. I guess the fix is to revert my save back, but that's days of progress lost (or kill one of the parents...). Kind of a bummer, but it is what it is.
I've also confirmed this doesn't seem to happen when the couple is opposite sex; it's only when they're a same-sex couple.
I'm also not using any CC or Mods (in fact I never have).
Hope that helps too! Good luck!
