Cannot set parents for pre-existing sims

I’m sure this was already said .. It just won’t let me respond but I’ve been trying to add parents to existing created sims I have.. Currently was trying too for my sibling sims Yasmine & Dominic who are brother & sister .. However, when I go to add the parents.. I can create the dad for example and it’ll let me list them both as his kids but when I go to create the mom .. it allows me to list only one of them but then removes the other parents & then the drop-down options change to like brother/sister or partner and I don’t want that.. I want to be able to go in at any point and create new parents. Can we please get this fixed to where we can get this ? I play on PlayStation so no cheats/no mods