Have your sister reset her passcode. You can plug it into iTunes and go to the help chat. If that does not work you will need to hard reset the phone. To hard reset plug the iPhone 5C into your computer with iTunes open. Get to the screen where it asks for the passcode. Hold down the "Power" Button at the top and the "Home" button at the same time. Keep holing them after you see the slide to power off device screen keep holding. Wait until you see the "Apple" symbol to show up on the screen. Then release the "Power" button but keep holding the "Home" button. Release the "Home" button when you a screen with a lightning cable and an iTunes logo show up. Then go to your computer it should say iTunes has detected an iPhone in recovery mode you must restore this. You must back the phone up before going any further. also it would help if your sister signed into her account. After you back up the phone click Restore. There may be an option to Restore and Back up the phone.
You will need a Pentalope screw driver it's like a torque driver just a bit different you will get one in the kit if you buy the kit with the tools. There are only 2 Pentalope screws they hold the front screen to the housing. You will need a suction cup and a hair dryer or an adjustable heat gun or s small sock with rice in it. The glue on the front housing will loosen easier when it is heated but don't heat it too much you can mess some things up by over heating your screen.