Is it obligatory to fulfill a vow to distribute food to the poor, which was vowed upon the death of the husband's father, knowing that he has indeed passed away, and had caused the one who made the vow material and moral harm?

If someone vows to distribute food to the poor upon the death of a certain person, this is a vow of obedience that must be fulfilled if that person dies, based on the Prophet's (peace and blessings be upon him) saying: "Whoever vows to obey Allah, let him obey Him." A conditional vow (nadhir mu'allaq) is disliked and forbidden, because it does not avert anything and is used to extract generosity from the stingy. Furthermore, wishing for the death of another person is subject to consideration: if the person is an oppressor or a corruptor, then there is no harm in that. Therefore, your sister must fulfill what she has vowed.