Hey there @AnneMF,
Thanks for reaching out here and welcome to the Community!
If you've already joined your sister's Family plan there's no need to cancel your own Premium subscription, as all subsequent payments on your account are made by the plan’s manager.
Keep in mind that any remaining Premium time you had will be forfeited once you become a member of a Family plan.
With that said, if you canceled the subscription yesterday and your account now reverted back to Free, you can ask your sister to send you a new invite to rejoin her plan.
For full info on Premium Family you can check out this support page.
Hope this helps. Let us know if you have any other questions.
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