How do we know that the mother is not happy with the current living arrangements? How do we know that he is not fulfilling his obligations, maybe they visit frequently? Was this guy old enough to work to pay for the education/marriage of his siblings? Maybe he contributed something, or maybe the maamu is very wealthy and offered to help out.
We just don't have enough information here to judge anyone. If the son was living with his family and his wife's family was living just round the corner in the same situation I doubt you would be posting this question.