Why do my cousins hate me?

I'm 15 and my cousins hate me so much and I don't know why? For instance when it is my birthday none of my cousins or my family wish me a happy birthday.But me on the other hand when it is one of my cousins birthdays I tell her happy birthday(all my cousins are girls).I never get invited to family outings with them and when I do,they dis include me in EVERYTHING so I end up just sitting there listening to my Ipod.They treat me like I'm retarded but I'm not and it really hurts my feelings.I'm like a circus monkey to them because when they tell me to do stuff for them and I do it because I hate hurting people's feelings.I think they all wish I wasn't their cousin.They never even talk to me,the longest they've ever talked to me was like 5 minutes.It hurts my feelings so much but I just suck it up and hold it in.I feel like the outcast in my family,I never did anything to them to make them treat me like this.I try to talk to them but they ignore me.