Is there a way to reactivate a sim that was taken away?

That's an interesting and intriguing idea! Also, it does make sense, I mean, if social workers come to take a child away, it's gotta grow up somewhere else.

Off the top of my hat, I think I'd start by checking existing NPC households and see if she lives with one of them.

At the very least, you should be able to move the gallery save of that family to a different lot, then age her up by having her repeatedly blow out candles on a birthday cake, until she's a YA herself. Then, possibly, depending on how skilled you want her, cheat those, as well as some pertinent traits to go along with the background you want her to return with. Once you move her out from the original family, you can evict them and have her begin the family story with her brother that you want. 

I've had a child taken away once, and that child stayed forever in the family tree, and did not age at all.  I looked for him in the NPC households but could never find him.  Since he was a tot I could never phone him either. Gone , nothing but a memory.

 If anyone had a relationship with yourmissing sim, maybe try calling her on the phone? But for some reason i think she is probably lost forever.

I think your only chance will be what  GameGirl2168a  suggested.

Thanks all! I created the grown up version by aging the album girl in CAS. A pity she is not having the family tree, but as I keep my own family tree I guess I'll be able to avoid their kids marrying etc. It was fun seeing her again, wondering how her teenage years turned out with her foster family  in Sim City. Her interests **bleep** a child  were always linked to nature, so I guess he did not like the city that much. Now that she is back in my nation, I guess it's time to reunite with her brother...

PS: Never looked for her in the unplayed households because her tree portrait was still a child, and I make sure to age all non played when aging up my played households. She would have been a YA by now if she was in any household.

Yeah, Life & Death pre-pack patch added the feature where the children taken away will not be deleted but instead appear in Manage Households. Children that were taken away before this patch are still deleted though and there is no way to get them back without recovering an older version of save. So yeah, recreating her as a young adult is the only way.

In addition, at one point, deleted sims (whether it's by being culled, released to netherworld, old version of being taken away by social worker etc) will only retain their last portrait, name, and place in the family tree (relations). This is sometimes important to know as if your sim already had two parents, you cannot create new ones in CAS even after the original ones are deleted. The system is shaky though, as sometimes patches may delete deleted sims from family trees (this has happened to me).

I'm glad you figured out a work-around, now you can have a family reunion.  Did you can make them brother and sister in cas with the cas.fulleditmode cheat?