Adding one parent removes another

I added parents for some of my Sims when the new pack came out, and with a couple of different families there's an issue where if I add a father, the mother is removed, and if I add a mother, the father is removed. Sim A has two parents, Sim R (father) and Sim I (mother). If I add R as Father, I is set to Stepmother. If I use MCCC or CAS to add I as his Mother, R is set to Stepfather. I have tried to delete his parents and re-add them several times and this same issue always replicates. The issue exists in CAS as well, one parent can be chosen as his parent but it always sets the other to housemate, with other options greyed out. There is an empty portrait added for the Sim's other parent, so if R is set to Father, A's mother is an empty portrait, if I is set as his Mother, then his father is an empty portrait also. It just see-saws like this instead of consolidating the family tree.