The original owner (your sister) must unlock the iPhone or the iPhone will remain locked and useless.
Turn off Find My iPhone Activation Lock - Apple Support
If your sister needs assistance with her Apple ID or password:
https://iforgot.apple.com/password/verify/appleid
If this does not help her, there will be no way to ever use that iPhone.
If this does help her and the Activation Lock is removed, then be sure to thoroughly cleanse the iPhone as described here:
What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support