my sister's iPhone was stolen

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery ( How to use account recovery when you can’t reset your Apple ID password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.

This document has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

This Apple article has instructions for what to do if you have forgotten your password --> If you forgot your Apple ID password - Apple Support