Thanks for reaching out about this in the Community!
Just to confirm, is your sister seeing an error message when trying to join your Premium for Family plan with her Spotify account? If so, it'd be helpful if you could send us a screenshot of what she sees. Just make sure that no sensitive or private info is displayed.
Keep in mind that all members of a Premium for Family plan must reside at the same physical address. If we’re unable to verify the members of a Premium for Family plan, for example if they entered the incorrect address, they will be removed from the plan. However, they can always restore their Premium features by upgrading to a standalone Premium subscription. They can also take a look at our other trials and offers.
Let us know how you get on! We'll be right here if you need more help.
Have a lovely day 🙂
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