It is really common, actually your situation seems to come up less frequently in this sub. Often the parents are busy with an autistic child and pay less attention to the sibling, or they're enabling a drug addict and the parents get angry when the sibling tries to set boundaries, or the parents have normalized a BPD sibling's behavior to the point of allowing the sibling to be victimized. These kinds of kids take so much extra time and attention, and the siblings often become self-sufficient, so if the parents aren't very careful the situation tilts, and sometimes dramatically so.

Parents like yours tend to be the ones who didn't realize parenting would be as hard as it is, so they favor the "easy" child. The athletic successful one over the one complaining - regardless of the complaint's validity. I'm sorry your parents suck. Every time my kids complain about each other and I can't tell where the truth lies, I'm convinced I'm screwing them up for life. I don't know how old you are or where you are in life, but if you haven't ever talked it through with a therapist it can help more than you think. It's especially surprising how much of your childhood crap will come back to you when(if) you have your own kids.