My older cousin is going out with my best friend, please help?

My cousin is 22, and my best friend and I are both 17. We were always really close, and told each everything like best friends do, but with my cousin I was a bit more distant, since he is older and we didn't hang out that often. Then one time when I was hanging out with him and our family, she came over to work on this school project and they met. Then I don't know how it even happened, but started dating? Now its just so awkward for me because they share a lot to each other, and some of it is about me.. Like, when I told my friend about some guy that I met that I was going to hang out she immediately told him and he got mad at me and started yelling which I don't understand... Ugh its so complicated for me, because whatever I tell her that's kind of personal she tells him, so obviously I stopped telling her stuff. Now I'm closer with my other friends, and she doesn't seem to care that our relationship deteriorated. All she cares about lately is him.. What can I do??