What will you do ?

According to me you better directly talk to her and clarify the things.You are only in doubt .As he is her cousin , there might be an emotional attachment between them rather than relationship. so before discussing be ready to accept either side,It may turn up your relationship badly or you may clear your doubt.

according to me u ask her straight wat u think .if she says dat she not loves u anymore den leave her. and if she blame u for keeping eye on her and not to trust her .den u must watch out wat her thinkings for the guy u doubted .this can be very painful if u not ask her imedeiatly dont bother abt after wards effects talk to her like man be a man