No, a child taken away by CPS can't just come home when he wants to. Usually, children can come back whenever the safety/parenting plan that CPS has put into place has been satisfied. You should hire a lawyer and discuss this with him/her.
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Seek the advice of an experienced Family Law attorney right away. CPS does not just willy nilly take a child out of his or her home, so you need to find out what the allegations were. Is the older sister minor? Is someone trying to get guardianship? You need an attorney.