Hey there guys, I have a problem and don't know how to react.

My sister is almost 16yo and has become fond of 23yo male, the same is true in reverse. They have been going out for two weeks. I don't like the age gap at her age. Do you think I should intervene somehow? Scaring of the guy is out of the question as another one can come. Telling my parents is not good because they will probably ground her and in the this will only inspire her to pursue the guy.

P.p in my country it is legal to have sex at that age and she is already active. They have been talking about sex already, but still have not slept together.

So what would you do? Let her learn her lesson by getting hurt or what?

Edit: I am 23 also, so I think that this is why I am so concerned as I cannot see myself with girl this young.