My Baby Cousin Flirts With My Bf !?

She is just like my sister and i tell her everything and last week i intruduced her to him and she seriously started flirting infront me X_x She says its just Sis & Bro way but It really hurts.. She even calls him huni or sweety ... I talked with her sister about it and we both think its wrong. It is wrong, right? I just dont know how to tell her that? Its really hurting.. Cuz of my exams she is becoming real close with him and i feel left out. How could i tell her without hurting her that what she is doing is wrong? And do i have the right? And if u were my place would u do the same or just nevermind her? Help please, she tells me how she flirts x_x he is older than her with 3 years and i love him a lot and he told me he doesnt like it But she tells me that he just goes along with her.. They are tearing me apart�