Son not taking care of his mother and bear the expenses.

My mother-in-law has been staying with us for 4 years since my wife's child birth. My mother-in-law has a son, now my wife is unable to look after her mother, so I asked my brother-in-law to take care of his mother but he is refusing now, the actual reason of taking my mother-in-law in because my brother-in-law does not have children and they told us they are trying IVF, so until the time they undergo treatment they asked us to keep my mother-in-law with us but now they are refusing, so I would like to know how to take this up legally, please answer.