What should I do to get this girl to stop?

I'm 12 and there's this girl 10 years old that comes to vacation with my family every year. My cousins fiance's sisters kid. She is from Arkansas and I'm from California. She comes to California to the beach with my family and she's annoying. She competes for my family members love and acts like shes better than me, like shes queen and I'm peasant. It pisses me off. And my cousin (he's old obviously) was always cool with me now when I talk to him he like looses all life and acts like he's talking to a lifeless stick when he's talking to me. No bond what so ever now. The girl was making me so mad at the beach I was gonna flip out and lose it (never done that before) she was really getting on my nerves bad, the little things she said, I got jobs to do around the house we were renting she got no jobs, and then one time she tried to tell me how to do mine. I was about to be like SHUT THE HELL UP. The whole time she was adding things like when I was driving with her and my uncle for the car show, we ped by my family on the sidewalk along with tons of others and they all waved and she was like " They were all waving at me and said my name" and in the backyard with my cousin she was like, (my cousins name is jeff) I will be Jeff's NIECE and you'll just be his cousin. My cousin and his fiance weren't even married yet for them to be related. I almost shouted YEAH WELL I HAVE BLOOD WITH HIM KID. she did stuff like that