My sister is acting strangely?

It could very well be that your sister is jealous of u love, maybe she notices the changes you have under gone lately and its making her insecure with herself. My advice to you is to sit your sister down, and remind her that you love her, and will always be there for her. Let her know just because your body has gone thru changes u r still her sister. Make sure she's know's she beautiful too. And if those little boys can't see that in her, they are not worth her time.