Have Been Feeling Like the Third Wheel. What Should I Do?

I have 4 siblings, but my sisters and I are the closest. We used to spend every weekend together, hang out all of the time, and tell each other everything. But lately I have been feeling like the third wheel. My sisters have been hanging out together a little more often, and making plans with out letting me know until the last minute, when I either have already made other plans or it’s something they know I won’t have money for. Sometimes when we go out I will pay or help to pay if one doesn’t have enough money. This is NEVER offered back to me. My older sister is 26, and my little sister is 17 they have boyfriends and are still boy crazy. Im 18, do not have a boy friend but am just as into guys as them. Last weekend they made plans and didn’t include me, claiming that they were “making plans for my birthday”, which isn’t even until June and I also told my little sister something in confidence asking her to please not tell anyone including my older sister. Of course she did and now they have been rudely picking at me and throwing it in my face.....we have been getting into little disagreements lately over silly things that never even mattered before. My best friend just moved back from GA, and they strongly dislike her and I don’t understand why. We have talked about it before but it always ends up in an argument. Since my friend has been back it seems they have been purposely avoiding me and try to get me to ditch her all of the time. It was not this way before she moved to GA and I do not understand why this is going on now. Is it just me? I have tried to include my friend in on things we do but they always decline. I do not always ask because I know as sisters we need our "us" time, but it is going to far....My sisters are getting closer and leaving me behind. What should I do?? Is it just me? Or are my sisters really leaving me out???