Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: It is perfectly OK to do so; I would go even farther to state that, if one was to do so, it will be reckoned as a great act of kindness on their part to offer money or food to the poor on behalf of their old relative. By making the niyyah (intention) of paying them in lieu of one’s relative missed fasts, this relative will not only be absolved of their duty to pay, but also will receive the full blessings.
Furthermore, the one volunteering to pay will also stand to gain blessings and rewards for their gesture of kindness on behalf of their relative. It is worth remembering that fostering ties of kinship is a supreme act of virtue meriting tremendous rewards.