I know you are upset but this is her daughters FIRST baby, she is excited to see her grandchild. You may not feel this way now but you may actually like having her there, she will be able to sit with the baby while you rest. I know she is disabled but she doesn't need to move around much since newborns just lay there. Plus, while your hubby is at work she will be there to keep you sane, she will have lots of good advice for you if your baby wont stop crying and you just cant take it anymore.
When I had Noah my mom was in the delivery room and she was at my house almost everyday after his birth, I wish she could have been there more if that is even possible. Joe was pretty good with helping but it was still nice to have my mommy, it was very comforting.