BobMarley There are many reasons, some of which may be me clutching at straws others may be true, I really try not to think about it too much anymore.
One main factor could be that he is never able to do wrong in my mum's eyes, my dad saw it but stayed quiet for an easy life. He is god and can do no wrong according to mum. My mum worships the ground he walks on and I am not kidding. I on the other hand did the most terrible thing of being born female and first, boys are revered in my family and girls are nothing more than a means to an end. I came to the conclusion a while ago that my mum has narc tendencies and I am the scapegoat while my brother is the goldenchild. I have worked hard at making my peace with that as they will never change or see what they have done to me and my mental wellbeing, so it is up to me to deal with how I feel about it all and how I deal with it all.
My mum and I have got on better since I have moved out, I was 17 and am now 33. I get on okay with her but have a closer relationship to my dad, I am cordial with my brother although he isn't to me 
It is sadly very reassuring that others have similar bad relationships with their siblings, it makes me very sad but helps me see that I am not the only one going through this and gives me strength.
I am lucky I have a fantastic husband and my girls are my world, my extended family are fab and I am very lucky I have them, also DH's family are fantastic and have accepted me into the folds of their family, warts and all, and love me as one of their own as I love them. Also my MIL is the most kind, amazing, funny woman I have ever met and she loves me for being me, something which my own mother could never have done and will never do.
SmilingandWaving I agree, it is very difficult to reconcile the feelings we have, I think I have almost mourned the loss/non existence of my relationship with my brother and have almost come to terms with it.