Trying to Cheat Add Grim Reaper as Parent but won't work

I literally can add other Grimborns (tested Nyon) or other members of the household to Son/Daughter (various age groups and household members) but my "main" who is a Grimborn, and I am going banana's.

I use to be able to do this but suddenly can't. Current save, new save, previous households/gallery households with my "main" (edit: works if I make a twin from her), Adding -> Grim to Family he will toggle son/daughter to every other Sim but my main, I changed age for both nothing works, I can only make him a romantic partner or sister and if I make her a teen I just an error.