She Stalks My Twitter And Facebook?

[Hello]

Whenever I put a new picture as my profile picture on my facebook profile, my older sister will say to me 'like your new profile picture its so posy' sarcastically. Then she'll continue to tell my mom and dad that I have a new one. Everytime I upload a status (which isn't very often), she'll read it aloud for everyone to hear and she'll take the piss out of it. It seriously annoys me. Why does she do this? She also looks through all of my photos all the time, making a verbal comment on each one but not actually posting it on Facebook.
I have a twitter account and have 200 or so followers, and she has one too but she only has about 70. Almost every night she looks through all my tweets, making fun of me and telling everyone. She'll look through my photos, saying 'god thats posy' and 'you're so vain'. She has loads of pictures of herself on twitter but apparently they're not vain because she didn't take them herself?!
For example, the other night me and my family were watching TV after dinner and she's on her blackberry and she'll go, "Lets have a look at your tweets." And continues to read out every one of them. What is wrong with her?! Do you guys know why she is like this?! Sorry I've been wittering on for a while but please post your thoughts and suggestions below!
Thanks :)xx