I NEED ADVICE PLEASE?

Okay so there's this guy, we are not actually friends but we used to be at one point, and him and my little sister are like "talking" now all of a sudden. It is sooo ANNOOOYIIING. They have been texting and she told me how he said she's cute and all and she sent him a picture. I guess you can say I officially hate this guy now. At certain points when she received a text from him she'd giggle or act surprised (as if he's talking perverted to her) and I don't like this AT ALL.She let me text him and I tried to sortive tell him to back off but he took it as a joke (it did sound kind of playful by the way) but I was still kind of serious, i tried to use my sisters phone to text him again to let him realize I wasn't kidding and now she won't let me text him or tell me what there discussing. I know its personal but I should have the right to protect her and the guy is a perv and just got caught cheating on this other girl. He is not the right person for my little sister and no matter how much I tell her she doesn't care and keeps acting like it isn't that serious plus I think she should stick to boys in her grade(8th) instead of boys in my grade(10th) I know its only 2 years apart but still. What should I do? I really hate these 2 flirting.