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

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.