Easy 10 points!!!! Facebook help!!!?

hello everyone, i'm gwen. I have an annoying sister who is in my friend's list on facebook...she's elder to me. She keeps visiting my wall again and again to keep a watch on me. I can't even block her as she has threatened me....I have a boyfriend too who is on facebook....I make flirty comments on his photos and posts but they get posted to my sister's wall and she comes to know all this stuff. I don't want my sister to see the comments that I make on my friends wall, photos or status updates..:( Is there an option on facebook to block a person from viewing the comments that I make on other friend's wall? :( Please help me!!!!!!!!