Cannot edit pre existing sims

I can’t change the household relationship of sims cause it says they’re pre-existing. But I haven’t clicked tick to go to world yet, only one sim is pre-existing and that’s not the one I’m having an issue with. 
I even tried putting Cas.fulleditmode on and it still don’t work so now I have to delete them all and restart.. which is long cause I’m just trying to make the cousins family of the sim but I have to make the mama and the auntie just to be able to set them as cousins.