Is it permissible to convince one's sister to annul her engagement and return to her first husband, from whom she had children, especially since the husband wishes to take her back?

The first husband, after his wife has been divorced from him and it becomes permissible for him to marry her, becomes a suitor. If your sister has accepted this suitor and they have agreed to the engagement, it is not permissible to annul her engagement for the sake of a second suitor, even if he was her first husband and the father of her children. This is because retracting an acceptance after it has been given to a suitor is forbidden if it is for the sake of a second su suitor.