apple id account trouble?

I've tried everything that I can think of to fix this problem, but it just won't change.

I have my own apple id, though my sister's (who has never used this mac) is the default login for the app store. She was logged into iCloud for some reason, so I logged her out and signed myself in. I also logged in to the app store, but when I go to update, her username is typed in and greyed out, so that I can't change it. I don't want to use hers, and haven't the slightest idea why she's logged in there in the first place. She's barely even touched my laptop. She does have a new iPhone 5, and innitially used my apple id to set hers up, but has since reset her entire phone, changing it.

This is really bothering me, as I can't make any updates to the programs created by apple.

Please help!

-Kate