How to deal with Bigger sibling?

I'm about 3 years younger than my sibling "B". B is much louder and confident. And i'm quieter and insecure. I like my sister. (well shes my sister I have to). But she always gets the most people to hang out with when we go to parties . Even the peoeple who Hate her still be nice to her because she will literally bite your head off if you start a disagreement. So everytime i go to parties i try to hang out with a girl my age wich is "S". but she seems more intrested in my sister. In fact everyone is. I just feel like a third wheel. I've just given up on trying to be friends. So now at parties i just sit there and listen to music on my Ipod. Also sinse i am a "little" nicer than her everytime these annoyinmg second grader come to hang out with us my sister yells at thenm to leave. and when they dont her and her other friends leave me with them.When i try to defend the second graders beacause they can come if they want to. they just end up teasing me like "aren't you hangign out with your friends" or "hey you should go up they're and defend them from us" and when another girl "C" came they just ditched me and left. I really need help. Any answers!!!