Is one obligated to fulfill a promise to protect the brother-in-law who assaulted his wife and abducted their daughter, in exchange for the return of the daughter?

If your sister's husband wrongs her by kidnapping her daughter or severely beating his wife, you do not have the right to waive your sister's right or prevent her from obtaining her right. His punishment should be carried out by the judge, because the establishment of hudud (punishments) and ta'zirat (discretionary punishments) belongs to the ruler, not to individuals. You must fulfill your promise as long as it does not prevent others from obtaining their rights. Whoever has a right should seek it through legitimate means, so that order is not corrupted.