Apple Watch

This support article has information about Activation Lock on your Apple Watch and how to turn it off as part of preparing a Watch for your use. --> About Activation Lock on your Apple Watch - Apple Support

This document tells you the various ways you can try to discover your forgotten Apple ID --> If you forgot your Apple ID email address or phone number - Apple Support Note that typically this is an email address you use.

This document outlines what to do If you forgot your Apple ID password --> If you forgot your Apple ID password - Apple Support

or go directly to this link --> http://iforgot.apple.com/

If you still need help:

Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Click here --> Contact Apple for help with Apple ID account security - Apple Support

She will first need to recover her Apple ID and password, she can try and enroll in account recovery

How to use account recovery when you can’t reset your Apple ID password - Apple Support

If this is not possible, and your sister is the original owner of the watch, and has possession of original proof of purchase documentation, she can start an activation lock support request

How to remove Activation Lock - Apple Support