Sims on family tree but not in world?

So I was playing a legacy challenge on the sims and one of the rules is for a child to be closer to their grandparents then their actual parents however they died whilst I was playing in a separate household.

So I thought, it wouldn’t be against the rules to revive the grandmother, I did and tried inviting her over the house which didn’t work since it was too late in sims time but then after 5 minutes she disappeared from everyone’s friends list?

so I checked the family tree to see if she just got deleted but she was still there on the family tree? So I tried using a cheat to make my current sim friends with her which didn’t work and even tried summoning her to the lot however said sim not found? 

I did the same with her husband, I revived him but he was fine, I added him back the the household easily with no complications but my grandmother sim has just vanished and I don’t know why or how? 

I even tried getting a family management mod to see if it helped, however when I tried summoning her with the mod, it said she was summoned but she wasn’t there in the lot at all. 

does anyone know what’s happened or know the solution for this? It shows she’s alive in the family tree but she somehow doesn’t exist in the game?