Can't Make My Sims Siblings

I downloaded my Sims from an older save file from the gallery. They were in different households, but when I temporarily put them in one household to fix their relationship to siblings, it just. Won't let me??  I tried just doing it in CAS, and I tried the MCCC mod to fix it, but neither would let me change their relationships to siblings at all. I even tried removing the MCCC mod to see if that was the issue, and nothing changed. I'm unsure if this is a new bug or if I'm just doing something wrong.