Recover my sister Apple ID password

The method used depend on several factors, but all of her options are outlined here:

If you forgot your Apple Account password - Apple Support

shatika146 wrote:

Still no help

So far, this iPhone is indistinguishable from a lost or stolen iPhone.

What access into the contents of this iPhone do you have? Do you have the iPhone passcode?

If you can recall some info about the Apple Accoubt:

• If you forgot your Apple Account password - Apple Support
• If you forgot your Apple Account primary email address or phone number - Apple Support

In Settings, in the top entry for your Apple Account, what Apple Account is shown?

Open Settings, Tap on your sister’s Name and Apple Account entry at the top of the list, and then look at the second line now shown; right below the account holder’s name.

That is the Apple Account.

From that, you can start Account Recovery,

Your sister would have to follow the steps provided here:

If you forgot your Apple Account password - Apple Support

NEVER post your phone number on a public website such as this. What can happen is that you may get a phishing email claiming to be from Apple where you inadvertently turn over personal/financial information. Anyone the reads your post here has the ability to do that.