I have family sharing set up with my wife but I can’t share her AppleTV account

If two accounts are part of of the same Family Sharing group, and one of them subscribes to Apple TV+, then both accounts can access the Apple TV+ content automatically. (The subscription itself stays with the original subscriber administratively.)

Each group member accesses the shared subscriptions and content on their own account. The account itself should not be shared.

Family Sharing - Apple

What you can do with Family Sharing - Apple Support

Set up Family Sharing - Apple Support

Sharing of iTunes Store purchases (‘buy’ content) requires the additional Purchase Sharing setting in Family Sharing, as set by the organizer. Consequently, with that setting, all group purchases are then billed to the organizer.

How to share purchases with your family - Apple Support

That is not supposed to happen, if one of the group members has an active Apple TV+ subscription.

At this time, I would suggest to contact an expert from the Apple Support staff for further assistance and investigation. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).