I don't have any solution for you. I wish I did. I have three siblings and was often treated the same way. I mentioned it to my mom once and she told me it was all my imagination so I attempted to just let it go and forget about it.....until my family including two of my siblings (my third lives too far away) planned a family Thanksgiving cruise and they didn't invite me. The only reason I found out about it is because my brother was having problems getting a passport and contacted me to help. How sweet is that!
I know you didn't need a rant on someone else's family issues. All I can say is you're not alone....