What would you do if you found out your 16-year old sister was having sex?

Well you said you don't want to break your trust between you and your sister so I think you should speak to her about first and tell her about your feelings of her having sex at this time. Then maybe you can start telling her some consequences of having sex and that it would be better for her to save her self for the right person when he comes along, but basically its up to her decision and not yours

I think this is a chance for you to be a really good brother.
I was 10 when I walked in on my sister and her boyfriend when she was 15 and I "dobbed" and my sister and I have just over the past year started to get along now and I am 18 now.
I guess its her choice just I dunno maybe casually mention something about protection.

By her age 16, I would have counseled her about sex. So in this situation, I would trust her because I know her well and I've taught her things.

In your situation, you seem to be a pretty good man, I suggest you talk to her and tell her what you think is right, and don't argue with her! Do you think he's a good guy? Share your wisdom with your sister and trust her afterwards. If she gets outta hand, it's better that you straighten her yourself, rather than parents.

Cheers ^ ^

Telling the boyfriend to get the heck out is not going to help; she can always find another place to resume the activities at. As I said in another post, not talking to your female sibling about the schemes guys do to have sex with girls, always leads to these type of situations. When girls are taught at an earlier age from their dads or brothers the games guys play, most of the times they end up making the right choices.

Two options come to mind as to what to do:
First, before confronting her about her activities, and since both of you trust each other, I would first try to find out what her mentality is about this issue. In other words, I would like to know first what led her to have sex at this earlier age. For example, at this stage I would be interested in knowing if she made this choice because she believes she loves this guy or because she just wanted to have sex. If the latter is the case, good luck.
If the first option is the case, then tell her that you hope she made the right choice with this guy. Then, you wait and see if this boyfriend breaks up with her, and if he does, then that's your chance to emphasize the lesson to wait for the right guy.

Second option is to just tell her that you know she is having sex, and ask her how come she made that choice at such an earlier age. Again, if she says she did it because she wanted to have sex, then not much you can do. However, if she says she is having sex cause she loves this guy, then I would educate her on what guys do/say to girls to get them to have sex with them; then hope he breaks up with her so that she can see your point of waiting for the right time and the right guy. Otherwise you'll get the standard 16 year old response that you don't know what love is, and that she is in love, etc, etc.

In both cases though, if she made the choice cause her hormones were raging, and she just wanted to have sex, then is kind of late to teach her what the right choice is. Also, not to break the trust with her, you can always tell her that you're a bit disappointed in what she did but is her life, and you just hope she made the right choice. Otherwise, she'll start alienating from you, once you start 'preaching' on her.