My sister locked herself out of her phone and doesn't remember her Apple ID or password. I can reset the password for her if I can find out what the email is that is attached to the Apple ID. how do I find this out?

Hello anapana3,

The following steps address what you're describing specifically, and should provide a solution:

Other ways to find your Apple ID

You can check if you're signed in to the App Store, FaceTime, or Messages on your iPhone, iPad, or Mac, or check if you're signed in to iTunes for Windows on a PC. You can also go to iCloud.com or appleid.apple.com and see if your Apple ID prefills on the sign-in screen. 

FaceTime isn't available in all countries or regions.

Check your inbox for emails from Apple

Your Apple ID might be included in some emails from Apple. Search your inbox for receipts as well as repair, support, or billing emails from Apple.

If your email address is masked (j*****@icloud.com), the number of asterisks might not match the number of characters in your email address. 

You can find these steps along with other information that may be helpful, here: If you forgot your Apple ID email address or phone number - Apple Support

If you still have questions, reach out to Apple Support directly, as this may require closer guidance: Get Support

Cheers!