Why do they always like my cousin!? :(?

Ok me and my cousin are very close but everytime we hang out w/ a cute guy they always seem to favor her ? I've been told I'm attractive and I have no problem w/ my looks I've been told I'm pretty and hot by alot of people not to sound cocky but it's true and I'm happy w/ the way I look but I know there is always someone out there that is prettier but even my friends said that shes not all that pretty cus she has this huge gap in her teeth and just seems idk has this feature that makes her unattractive to them but everyone also says she has like a stuck up look if you don't know like she's better than everyone else but point is guys always choose her! Personality wise we are the same almostish x) shes plain Jane you know can keep up the conversation at least this is what her bff told me she also told me that I'm more random and not afraid to do things in public I was like okay? But gah it bugs me I always end up being just a side kick next to her like the girl that makes her look better by comparison on my own I get told I'm pretty and guys come up to me all the time but when I'm with her and if I really actully like the guy they go up to her! :( what's up with this I always end up getting pushed over when she talks to a guy and I just feel akward and like a third wheel and she will I know I like a guy and go up and flirt even when she has a bf and they flirt back of course cus she is kinda pretty but blah I hate it this has happened countless times before and I'm sick of it question is am I jealous? How do I get over this? Why do guys go to her? It's just the fact if I like a guy she can get him and not even care about him as much as I do and lead him on when she has a bf that bugs me the most and please don't say cause of boobs cus I have bigger boobs than she does even though they are not that big just a b cup and she's an a cup even though she does show the little cleavage she does have when we go shopping or something where as I wear a t shirt or