how to give already made sims siblings/parents {solved}

If all else fails, take the household into cas.fulleditmode, and if it still won't let you edit properly, save them to your library. Then add them as a 'new' household into the game and you can edit them in CAS. I just had to do this recently. Worked like a champ.

I did this with Vlad. Bring up the cheat box before you go into CAS and type in cas.fulleditmode. Then go into CAS to edit your sim. What I do is to create a child with the sim whose DNA I want (so, in my case, Vlad) and an unknown parent. Make the child a teen so you can see what they'll look like. Once you have someone whose look you like, then you can change the relationship so that the new sim is the parent and your sim is the daughter or son. Then your sim will not only have parents, but they'll have parents who share their DNA.

Yes, you need the cas.fulleditmode cheat before entering CAS. I was able to adjust family relations some times, while other times it did not work. So, if you run into trouble, consider saving one sibling to your album, then add to household. All skills/relations will be gone, but I think it might be easier adjusting family relation to a fresh sim. Maybe you did not play all those siblings as much yet.