Is it permissible for a mother-in-law to ask her daughter-in-law for money to fulfill her vow, and will the reward for the charity be for the daughter-in-law or for the mother-in-law?

The aforementioned vow is disliked, but since she has vowed to slaughter a sheep, she is obligated to fulfill it from her own money, not from the money of others. However, if someone else willingly donates the price of the sheep to her, then that is permissible, due to the generality of the Prophet’s (peace be upon him) saying: "It is not lawful to take the property of a Muslim person except with his willing consent." If the sister gives this amount to her mother-in-law as a willing gift, she will receive the reward for the gift.