PLEASE HELP! My Younger Sister Is Having Se*x Already. What Do I Do? Please friends I need your advice

PLEASE HELP! My Younger Sister Is Having Se*x Already. What Do I Do? Please friends I need your advice.

I'm 26, I don't usually go through people's phone but recently I stumbled on my 18 years old sister's phone. So I went through her whatsapp chat with a guy I've been suspecting she's having something to do with. 

Earlier this year I advised her about relationships generally when I found out about him. Now going through her phone I found out she's having sex with him now. 

What do I do? Should I confront her? Or just keep quiet and allow her experience life? Considering the fact that I have been advising her and if she wanted to take my advice she would have, and no matter how much I talk to her if she won't take it she won't and so she'll still do it. 

Note:

She's not really stubborn, she doesn't even go out much. She's going to 200L now and she's really doing well (a 4 pointer). She's really fragile even though she looks strong on the outside, and from their conversation the guy is not serious with her, I just don't want her to get hurt and get distracted from her education.