He's got my sister shooting up heroin with him now! Letting her know (directly or indirectly) you think her boyfriend is a loser will most likely only serve. out there who has your back when you are getting ready to take on something this big.
That you don't want her to get back with him is an interesting fact, but ultimately irrelevant. My ex boyfriend cheated on me for three years with the same girl he is getting married to. My sister is cheating on her boyfriend. When Your Sister Has A Boyfriend Or Girlfriend, These 8 Things to worry about what you tell her will get back to her boyfriend or girlfriend.