What should I do about my secretive younger sister?

[Jessica]

I am 25 and have a 16 year old sister. She is very secretive about her personal life with friends and boys, etc.

She has been especially secretive on social media sites, blocking myself and our entire family, so I know she is hiding things. Recently, she blocked me on Instagram and claims every time she does this that her account is messed up or has been hacked. I know this to be a lie. No one else is blocked and can see her posts. I looked at her Instagram through a friends account and found some disturbing posts including her with boys in bed and a mysterious cloud of smoke. We were not raised to act this way and none of our family is into drugs or promiscuity, etc. not saying she definitely is, but it looks that way in her photos.

Next, I found a picture of a painting I did for her for Christmas. It was very special and I put a lot of Time, thought and love into it. She followed it with a caption "just finished.. Not the best but I just wanted it to match my room" I made this painting for her specifically to match her room, and she's taking credit for it. All of her friends are posting comments about it being cool and amazing, and she keeps replying "thank you so much!" This is disturbing to me, yet extremely hurtful. I feel betrayed and insulted at the same time.

Now, what I'm asking is advice on how to handle this discovery. She doesn't know I know, and I always refrain from confronting her about things (such as stealing my clothes and posting photos of her wearing them, stealing my mom and other sister's things, having a secret boyfriend, etc.) because she gets embarrassed very easily and I do not want to embarrass her or upset her. I would just drop this but it has gone to a whole new level and I feel she is going to continue down a bad path if no one ever confronts her, because we never do, or calls her out on it. She never has to suffer consequences for her actions.

So, how should I handle this? What should I do?
Thanks for the advice in advance!