How can I tell my sister her boyfriend is always making pes at me?

Well my sister and I were close when we were younger but when I went to college she had a rebelious spurt also I was once engaged and she never liked my fiance' this caused a rift between us, but when I left the ex or yet when he left me things began to get better... But now she has a boyfriend and I once stated that I would never tell my sister if I saw him cheating but I would lead her to finding out.. I dont start trouble but anyway hes always saying Lee youre really pretty and he smiles at me behind my sisters back... once he even rubbed his private area and mouthed at me to "ride it"... now I know if I tell my sister she would swear up and down I was just being spiteful I try to avoid him whenever he comes over but he seeks me out to mouth and do all these provocative gestures.. I know my sister she will swear and she'll think I want him and stuff.. Last night he came into my room and grabbed my thigh and tried to rub my intimate area.. Im fed up thats ual harrasment