I would be honest! Tell her that she is always criticizing you and not contributing and that its time for her to leave! If someone is negatively affecting you and your son AND mooching off of you then they have to go!
I agree completely with donrae. If they aren't financially recovered 2 years later then when will they be? When will they be able to move out? My relationship with my in laws is good and my mom and husband get a long but I'm sure that it is a strain on your marriage to be living with your...