I found out my younger sister is making bad decisions should I tell my mother

First time using redit. So hopefully I can get some advice. So basically I found out that my (I'm 21)younger sister (she's 16) is had sex with a 18 year old. She, in the past was in a relationship and has told me she had sex with him too. But they have broken up. She has recently started going for walks with a guy saying they were just friends. It has been about a month or so. And she said last week something happened when they were out. ( She isn't on birth control, but apparently wrapped it) I had a chat with her and told her how stupid it is that she shouldn't even be doing that at 16. She told me that she knows that it is a bad decision but I said if you know it is a bad decision why would you continue to make the bad decision. It doesn't make sense to me. I am wondering if I should tell my mom even though I would be betraying her. But I would hopefully stop her from making bad decisions. I would hope my mom would stop her from going out. But I am not sure she would. I have told her before that I have caught her smoking joints and Mom hasn't done anything. When I had a chat with my sister she was laughing under her breath. I told her that it isn't funny. I know some of you could think I am being a prude, but seriously this is disgusting and controlling. But this makes me feel sick, I shouldnt turn my back on my sister but I don't want to be around her.

Please I need some advice. I need do something I think. Part of me thinks I should tell mom in front of my sister so she can't back track and she has to face it.