Not sure what are your needs but perhaps you could consider getting a part-time helper instead? Those who comes in once or twice a week to help clean the house. Living together is always tricky and would bound to bring about friction. Hence, if you are just looking for someone to help with the household chores, part-time helpers could be an option to consider.
However, if you need someone to help with taking care of your mother-in-law and children, I suggest you let your husband handle your mother-in-law when it comes to issues related to the helper. He would know better how to handle his mother and you could minimize unnecessary arguments with her. Best not to let the helper strain your relationship between you and your mother-in-law. Hang in there!!