I feel my family isn't..."my family"

I am an 18 year old male. I live with my grandparents, as does my younger brother, whom is 11 months younger than I. My half brother is a drug addict, as well as my dad...My two other full-blooded brothers are also drug addicts, and have been kicked out/moved out of the house. Only my younger brother and I have not "screwed up" with drugs, etc. getting a rap sheet record, etc. We are both intelligent, and maintained a 3.5+ in High School w/o trying. Our Grandmother is the only person in my family (in the house) I feel really gives a damn about me, for the most part. My homeless father cares, as does my mother, who can't stand 90%% of our family either.
There are quite a few incidents where I seem to get..."left out" and "used" by my family. My aunt, whom lives a few blocks away, is always saying how I will end up like the rest of my brothers/father. She's always scheming something up, and always has had a problem with me since I was a child. That sucked majorly, as my cousin was my best friend, and he hates how his mom does that to me. My grandfather will always leave me out of everything, or jump on my case if something doesn't seem to be working out, like having certain friends as a freind, etc, BUT, he will act all happy and glad when I pull up in my car and tells me (95%% of the time he does NOT ask, he TELLS me) to go do this, pick this person up, run my little brother all around the state. I get no gas money for this, or anything I do. My brother gets allowances at the age of 18 and does NOTHING but school and video games. Wtf? When a couple of my family members will gather at Christmas, they talk to me, etc. and give my brother cards/gifts/money or w/e, and then I get nothing except from my mother and girlfriend. I am not materialistic, so it doesn't bother me much, but I think it is wrong to aknowledge us both, give one gifts, and act like they didn't know the other got nothing. It's the thought, not the materials that bothers me. As for the car thing, I BOUGHT the car from my grandmother, and the title is in my name. So that's not right. Hmmm...Well, I had a few more things to give as examples from recent/times or the past, but with the train of thought, I lost the rest...Anyways, they ALWAYS do this to me, except my grandmother. I have stayed with my girlfriend (fiance...family doesn't know that) for a few weeks now, and the only reason most of my belongings except clothes are at my grandparents' is cause my grandma refuses to let my grandpa throw it out with me, and there is no room where I Stay now. So WTF is up with them? Any suggestions? I'd say "Am I normal..." but I have done nothing wrong...My brother is almost a mirror image of me, but is treated like a King of Kings and I am dirt. What should I do besides continue with the severance of family ties except to those I care for now?