My brother has adhd how am i meant to cope?

On monday, my 6 year old brother got diagnosed with ADHD, we have known he has had it since he was about 4 but they wouldn't test him until saw my sister on her 3 year heck up, they asked mother where she got her black eye and bruised back from, she said, joshua (my brother) every day i am called every swear word under the sun by him, i'm hit, kicked, slapped, punched and bitten, today he urinatied over my other brother, which is a step too far for me, so i went down stairs and got really angry with my mum about it, and she started crying and said if i really hated it that much here then i should move out, i feel really bad now and can't stand it making her feel like this, but i don't know what do , i just don't understand adhd well enough, this subject is reall upsetting me at the moment and i don't know what to do i just don't find it fair that it is never his fault but mine if he hits me even though i do nothing to provoke it, this won't help to not dissipline him for doing something wrong or when he gets older he wont understand that he can't do bad things an get away with it........ please i need advice, can anybody help?????????? thankx alot becki (14)