Why I think my sister is hiding something?

My older sister and I get along better than me and my little sister. But, first of all, my older sister is 15 years old and she done a lot of stupid things in the past. Such as, bringing her boyfriend over when my mom was at church. Especially, when he left out of her room, her room temperature was hot. I can't help to think she may had sex because ever since that day, she always has a sexual remark to everything.

It makes me think every remark she has might been what her boyfriend possibly done to her. One time, I found a picture of somebody's butt on the family computer. She claimed to be her friend's butt and send it to her ex, but I know she's lying. And my aunt found a Badoo account that my sister own and it listed her favorite things to do. And she was the most friendilest person on the app.

But, my sister had became anti-social since she was 11 and she keeps locking her door all the time. I can hear her laughing her ass off in her room, she always talks to her bestfriend. She always be the reason why our light bills to be high because she will stay on until about 1 or 2 in the morning. I think she's doing wrong because she would change her voice when somebody knocks on her door or she ignores everything. I know she broke up with 17 yr old boyfriend who had she dated three time (breaking up and cheated twice), he even has police charges and she doesn't even care.

I don't know if she's hiding anything from everyone. She always tell her secrets or who she has been texting to her bestfriend.
Why I think my sister is hiding something?
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