Is it permissible to give a relative who is in debt money from a vow (Nathr) or charity (Sadaqah), or is helping him obligatory due to kinship, and thus the money must be given to someone else?

If the poor person is someone whose maintenance is obligatory upon you, such as one of your parents, then it is not permissible to give them the obligatory vow, because it would be a protection for your wealth by what is already obligatory upon you. However, if the poor person is not someone whose maintenance is obligatory upon you, then there is no harm in giving them the vow; in fact, they are more deserving than others as long as they are in need.