Hi mgiroux236,
welcome to the Apple (user-to-user) Community.
Re: "not able to add member into family sharing"
Thank you for posting your query in the community, and for adding your info. about the situation.
You may need to update your iPhone to the latest iOS version:
About iOS 16 Updates - Apple Support says iOS 16.5.1 is the latest.
If you have already updated:
Try restarting your iPhone (deletes temporary files, and will often restore order or update recent account changes)
Restart your iPhone - Apple Support
Check again, whether you can now add the other family member.
To add another member to your family, you can send an invitation via Messages, email, or in person.
They can accept or decline an invitation to join a family directly from their device.
Set up Family Sharing - Apple Support says: Settings > Family > Add member > Follow the onscreen instructions.
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If necessary:
You could try checking if the step-daughter's name is still listed for family sharing anywhere else in settings
eg: Ask to Buy ? ScreenTime ? Apple cash .. maybe shared subscriptions ? or purchase sharing ?
More support, if needed:
See a list of Apple phone numbers around the world.
Contact Apple for support and service - Apple Support
Family Sharing website
Family Sharing - Apple
All the best :-)