There is no simple or clear answer. While the disabled and their care givers have considerable rights in regards to housing, they do not have a blank check to infringe on other people's rights. Depending on the circumstances and what can reasonably be done, it may be necessary for the girl's mother to take away whatever she is banging on or restrict her ability to bang; or for the landlord to make some accomodation, such as to put in much better soundproofing; or it might possibly be the case that you do have to move, but may at the least will have grounds to terminate the lease without penalty and possibly even receive some monetary compensation for being "constructively evicted" as well. You should speak with an attorney, who can evaluate all the specifics of your situation, and help you determine what recourse is possible.