Your sis has to sign in at iCloud.com with her Apple ID and password to remove the iPad from her Find My iPad. Her Apple ID is an email address. The hint on the iPad should help her figure out what that was. Then go to this Apple page to recover her password.
https://iforgot.apple.com/password/verify/appleid
If you are trying to activate an iPad or iPhone and it is asking for the original owners Apple ID and password, you have encountered the Activation Lock. This is a security feature that prevents thieves from setting up and using a stolen or lost iPad or iPhone. You have no alternative. No must have the original owner remove the device from their iCloud account to use the device. If this cannot be done return the device to where you got it. You will never be able to activate the device and no one can help you do it.