Brazilianbull mastiff attacked my 8 year old boy today?

my brothers dog attacked my 8 year old son today she punctured his lip and left two teeth marks down his forehead he also has bruising on his cheek, and is pretty shaken up about the ordeal. it's not the first time she has attacked, she went into a frenzy with my brother's pure staffy and cost him $2600 at the vet to get the staffy fixed up as she was torn up pretty bad. i said the mastiff should have been put down then. my brother was going to shoot her and everybody said that she was not going out like that, mum made an appointment to do it at the vet but forgot about it and the mastiff has been happily living at mum's with the staffy, a border collie and my three year old nephew. when she attacked my son today my brother just brushed it off and when i told mum she HAS to be put down she said that my son might have done something to provoke her and changed the subject pretty quickly. I know that if it had been my nephew that was attacked the dog would have been shot right there and then. so my question is...how do i get mum to put the dog down without pressing charges on my brother or mother or stealing the dog and having it done myself?