Family Sharing, Apple One, Not Working

In what way exactly does it not work? She does not receive the invitation? You can't see her in Family Sharing? She can see it and everything is set up but the app does not work?

Your sister has changed the country settings on her account, logged out and back in again and restarted her device?

Change your Apple ID country or region - Apple Support

Typical solutions for issues in adding a person are:

Try signing out of your account and back in again.

Try restarting devices involved.

Contact Apple and ask if they an tell you what this means, particularly if you see it repeatedly over a day or so:

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:

"See a list of Apple phone numbers around the world."

Click here --> Contact Apple for support and service - Apple Support