iPad is locked, little sister forgot her email and password what to do now?

Hi there,

Last year my brother bought an iPad air for my sons birthday, but bc of the some family related problems I must give it back (yeah sounds not correct)

After close to a year my son got it back but now we have an issue with the icloud thats been forgotten by my little sister and nobody but litteraly nobody remember the email and the phone number thats been used.

The iPad is still under warranty, is that a good option to go to the shop where its been bought and ask to send it to apple or is there any other solvings to it?

Thanks atleast for the answers and sorry for the battle languague.