What should I do about my secretive younger sister?

[Riley.]

Tricky situation Jessica! i can definitely understand how hurtful all these things are. It can seriously affect your relationship with your sister! You finding out that she posted those pictures isn't betraying her. It is not like you broke in to her account, you obviously did not believe her "hacking" lie and wanted to know what she was hiding.
Confronting her is the right thing to do. The important thing is to keep the parents out of the situation for now. You are her older sister, and she looks up to you surprisingly. She loves the painting and loves your clothes, but seems desperate for some good attention. I can guarantee she would be embarrassed if she found out that you knew what she was posting, and with good reason, she knows she shouldn't be posting it! I think the best way to handle this is to find her at a good mood, and just bring up the situation as "no big deal" as possible. Not that you're mad, but that you know. She may be upset or angry, but she shouldn't be doing many of those things and she'll be angry that you've found her out. This is when you get to the bottom of things, I really think she needs someone to talk to. Tell her you wont tell your parents, but if she wanted to talk about thinks that you are always there to talk. That you "don't have to steal my clothes" if you just ask, and that it hurt your feelings that she would try to take credit for your painting, I'm sure she hadn't thought of how you would feel because she wasn't expecting you to find out!

Good luck!

riley.