Ideas
something similar happened to my game a while back. i noticed the issue in the family tree first:
somehow - my sim (Bella of all people) became the daughter of one of my other sims. they were already married for... forever practically. it just happened one day.
just tried to merge the family and edit the household relationship, so her faux father was a household member. it was not possible. i was given the option in fulleditmode to select household member instead of father, but when i clicked on household member it doesnt switch. it just goes back to father.
tried to do the same thing with my sims husbands mother so she's no longer my sims sister - and same thing. it will let me edit other members in the house - but not the one thats the problem