Apple ID of a deceased family member

Some “Expectation Management”

Without knowing the iPhone’s passcode, the data actually residing on the phone will forever remain inaccessible via the phone itself.

Apple can’t even access it.

Next …

What Apple can provide is access to your sister’s iCloud. If your sister managed her phone as many (if not most) do, much of the info on her phone is likely replicated in her iCloud, which can be made accessible.

Finally, Apple can also release the Activation Lock on your Sister’s phone.

While this unfortunately will not provide you with direct access to your sister’s on-phone data …

… it will allow you to reset and re-purpose the physical device.

Best of luck with this task in a difficult time.

Sorry for your situation.

Your sister’s Apple Account is in almost all cases her email.

Here is your guide (as you’ve already discovered)

Note that because of the legal documentation required to be submitted to Apple, this is a task generally best handled by the estate executor.

Request access to a deceased family member’s Apple Account - Apple Support

You need to know the email address she used as her Apple ID, which by definition is the one logged in to her device under Settings ➜ Account.

That's the email you want to provide for the "Request access to a deceased family member’s Apple Account".

Do you have access to her email addresses?