You see your sibling

So I have kinda come across this situation when my sister was flirting with a guy she once had a thing with and her boyfriend wasn't in the city. At first I thought I should do something before she regrets anything, like tell her to come with me and interrupt them. But then I thought, oh well, she's going to be a fucking adult soon and I'm going to be away from her most of the time and I won't always have the means to hold her back from doing bullshit. So I just let her do whatever and hopefully she'd end up okay. The next day I told her she needs to be more mature at some point in her life and she agreed.