Is a husband forbidden from receiving the reward for his wife's breaking the fast of his mother, who deliberately breaks her fast alone and then joins them for the meal?

May Allah reward you for the good deeds you perform. Preparing food for your household earns you a reward, and it is hoped that you will be recompensed with the reward of assisting in breaking the fast of a fasting person. Your husband will also be rewarded for the food he brought. Your mother-in-law's deliberate breaking of her fast with something other than what you prepared does not harm you, for feeding a fasting person is achieved, according to a group of scholars, by providing them with enough to satisfy them. This opinion is the chosen view of Sheikh al-Islam. The reward for feeding your household is obtained, Allah willing, for your husband through what he earned, and for you through what you cooked. Even according to the second opinion, Allah will reward you for your intention and good aim.