i am 18F and my sister is 16F. she plays basketball and during one of her tournaments she met this guy who is 23. they've been talking for a while but I just assumed it's friendly or like a mentor of sorts since last I knew, she had feelings for someone else. but he recently came to meet her and she told me they have feelings for each other. he is a grown up man (clearly not if he's grooming kids but you get my point) and my sister is a kid. atleast to me she is. and idk what to do about it. one side he says he's not gonna date her because he's "not right for her and she should move on" but on the other side he still keeps talking to her and voices his feelings very loudly. my sister says he understands her and she's never been loved like this before. i myself have been a victim of grooming and pedophilia and it fucking sucks. i live with the guilt and disgust on my own self every day. i do not want her to go through that but I don't know what to do. I've tried talking to her in a good kind way aswell as the stern way but she just won't listen. I've talked to my parents about this aswell but they are just as helpless. idk what to do, any suggestions please? (ps no I can't report him)

edit: this blew up I really wasn't expecting it to. and I'm not able to reply to people idk why. i realise I made a mistake tagging it paedophilia but y'all get the meaning. to everyone asking me to tell my parents, my mom knows and she's just as helpless. also no I don't live in the states and it's not legal for a 23 y/o to have any romantic relation with a 16 y/o. i can report him, but there'd be consequences (my father is an abuser and if he finds out any of this is going on in the house all three of us are fucked [mom sister and me]) and really if they haven't had a physical relation the authorities cannot take any action. there is no use telling the school authorities or the coach because they literally wouldn't care (I live in a country where age gap marriages are awfully common). i have told her about my story just to make her see this as it is, but I think she took it as an invitation to prove me wrong by making me see how "good" he is and that they can "make it work". i wouldn't be this concerned if she was 18, but she's 16 and I do not want her to go through the same things I went through, because trust me I'm disgusted by myself for it every single day still. also whoever is saying this is normal needs to shut up because it's not and if you think it is you should probably count yourself one of those who likes young children.