Can some one give me some advice?!?!?

K well my mom prommised my brother that if he kept the house clean that he would get Guitar hero 3 and well he DID a very good job for 2 weeks and then when he was supossed to get it my mom said he didnt do a good enough job... my mom ALWAYS does this to us where she promises and the takes it back.. i feel bad for my brother what could i say to my mom to changer her mind or somethin?? Thanks :)