My story is different but I do get what you're saying. I believe my mom resents me because she got pregnant at 19, was forced to get married to my dad, and by the time I was two was divorced. When I was 6.5 my sister came along. She is/was will always be the favorite child. She is spoiled and bratty. She however, is my best friend, now..I actually couldn't stand her before I was 25. My relationship with her has always been rocky/stormy and always with drama.
Currently, my siste lives with me, pays no rent, no utilities but does use her foodstamps to help us with groceries. But she gets a weekly allowances from mom and dad (my "other dad"). She's going to school full time so I keep that in mind but it drives me nuts that they just do those things for her. My mom is "fair" with the grandkids for the most part, but my kids often "get left out" of certain activities because I don't get told until it's too late. Then I'm the bad guy cause we have other plans.
I am the "fixer" of problems with the entire family. When someone needs something to be fixed (emotions, friendships, arguements) I'm the one they call. I'm the one who never fails to let anyone down, but I don't get the respect or love I deserve. I'm 34, and still look for that.
You have to realize like I am starting to that it's not about you, it's about them. All I can do at this point is make sure that my kids NEVER feel this way.
Added: Then I look at the man sitting next to me, my husband the father of my children and think...I could easily have the bio-dad he does. Calls once a year, makes promises to be more involved, never keeps those promises. Has never met his grandkids, and probably never will. A man who was an alcoholic, who abused his wife and kids, and then when she remarried a wonderful, gave her money and signed his rights over.
I realize, I don't know what that feels like and I'm luckier than him.