my sister issues

Is it normal that my sister always yells at me of everything, always try to make feel like I don't know what I'm doing with my life and currently she is jelousy of me having a healthy normal friendship with her current boyfriend? She told me 10 min ago that I should stay away from him and do not try to be friendly with him, that I should respect her. That sister's boyfriends are forbidden and we shouldn't have too much contact with them. But I don't get it? Sheis friendly with mine, and she is always talking to him...  Im trying to be friendly because she is my sister and I care about the ones she care . I don't understand why she is so upset, I help her a lot with her daughter ( who is not the daughter of the one she is dating), she works all the morning so right now I moved my schedule to help her to take care of my niece in everything, and I'm trying to be a helper, not a trouble maker.... I replied to her telling that I understand her feelings and I'm going to try to stay away of her relationship but, I don't understand.. When she was married with the father of her child, she completely kept him away from us , the family , and she forbid us to talk to him... What's going on with this?