Sammy, I won't even pretent to know what you are going through. I do know though, that you need to grieve, and you haven't done that yet. My first response is to tell you to talk about it with a threapist, but I know you so well, and you won't.

You're so stubborn!

How about talking to your grandparents about her? Maybe your brothers who knew her better? If they could tell you things about her that you don't know, give you some picutures...something that you can actually grasp at. You are feeling a loss that you didn't even know was there until recentely. You have to find an outlet for that. You need to figure out, where in your head and heart it belongs.
You know, I'm no therapist, but you're the best friend I've ever had and I'll always be here for you to talk too. You are my sister.
Maybe visiting her grave site would help put some closure on this for you? Go by yourself, talk to her, ask her questions. Cry, hollor, scream your lungs out, but get it out Sammy, before it eats you up. I think if you can figure this out, a lot of the other problems in your life will get figured out as well.
I'm here for you, day and night, night and day. You know that, you can call me, write me or visit me. Whatever you need, I'm here.
