What should i do about my sister's love life?

last night was crazy my friend hosted a party at her summer house, and so my twin sister and i went to it and lately i have been concerned for her, she's the same age as me but her boyfriend is seventeen. two year age difference and he's been known to be a player and better yet he asks girls for sex, i never told my sister this because last time i told her something like this she flipped out and ignored me for three weeks, she's the opposite of me naive and shy so she doesn't like to be told the obvious... i know they have done something like a b job but not sex. should i tell her that this boy could be playing her they have only been together for a month. Should i tell my baby sister that this might just be lust and not love?