If you got an error message, saying there already is an existing account with her email, then it's likely you were able to finalize creating the account, so to access it, just try resetting the account's password and she should be able to log in.
As for the Premium plan, make sure her account's country matches yours, as it's necessary in order for her to be able to join the Family plan. You can check the country settings on the account page. In case they match, you can try updating the address on your Family account, then sending her a new invite and entering the new updated address for her account as well.
Let us know how it goes. Cheers!
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