Hi jimmi64,
Thanks for contacting Apple Support Communities!
We understand that you need help to update the information associated with your sisters Apple ID, correct?
Is the phone number you used to create the Apple ID or is it set as the trusted device to receive verification codes?
If the phone number is the Apple ID, you can change the Apple ID as outlined here: Change your Apple ID - Apple Support
"How to change your Apple ID to a different email address
- Go to appleid.apple.com and sign in.
- In the Sign-In and Security section, choose Apple ID.

3. Enter the email address that you want to use as your Apple ID.

4. Choose Change Apple ID.
5. If you changed your Apple ID to a third-party email address, check your email for a verification code, then enter the code. Learn what to do if you didn't get the email.
6. If you use services like iCloud or Messages to share with friends and colleagues, sign in to those services with your updated Apple ID."
If the phone number is being used to receive verification codes, you can use account recover to change the phone number to receive verification codes: How to use account recovery when you can’t reset your Apple ID password - Apple Support
If you need additional assistance with your Apple ID, you can reach Apple ID support here: Apple ID Support
Take care!