Agree. Just say to mom, I apologize but XXX is not fitng in well here. He tends to act out and doesn't listen and hits me and the others kids. Ive tried to make it work but unfortunately I will be letting him go as it is not working.
And don't feel bad or guilty. As much as we like to help all the kids, we just can't. Fill his spot with a child that will respect and love you as you deserve. You don't need that stress.
Good luck!