Although some people talk about Apple keeping data for a short while, I think what you may need to do is make sure that you have continuous coverage while doing this. This might become fiddly with you having to subscribe to iCloud while part of her family, then you leaving the Family and upgrading to Apple One, then her keeping the current upgraded iCloud storage while disbanding the old family after you leave, then you inviting her to your Family when she no longer has her Family going, then her discontinuing her iCloud storage (if it doesn't automatically cancel it when she joins you in Apple One).
What happens to your iCloud storage when you sign up for Apple One - Apple Support
If you still have questions or problems, or want to check if I have it right:
Contact Apple. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support