im signed in with apple id but my sister id pops up when i try gto purchase app

If you're trying to purchase directly fom your device (ie: iPhone, iPod, etc.), try first backing up your data then go into Settings--General--Reset--Reset All Settings. Then you can restore from your last backup and put in the correct Apple ID. Unless this I a device the both of you are sharing, then that's a different story.

Just go into settings, app store and sign out. Then sign in with your id. No need for a full restore.

@clarkkent27, its not a full restore. It resets all of the preferred settings.

Oh yes. My bad. That's the 2nd time i have misread a post tonight. Must need glasses lol. Sorry dude.