I think my step-sister is trying to become me... 0_0?

You should probably be flattered. You don't imitate someone that you dislike. You imitate someone you envy. She obviously admires your look and your appearance. She may also be trying to indicate that she is related to you by genes, rather than marriage. As to getting your stuff back, the easiest way is to tell her that you want it back, or, if that doesn't work, every time she borrows something of yours, borrow something you know she is attached to. Also you could borrow something of hers and don't return it. She'll get the message if it is something she cherishes.