What is the error message they are getting?

failed to accept the family invite you are ineligible to join this steam family at this time as youre steam activitu doesnt indicate you are in the same household as other members of this family

Try to login to the SAME PC with your Steam accounts.
First your family member, then you and then resend the invite again.
It should now work since Steam checks the login history of accounts that get invited to a family and they need to have logged in at the same location/PC at least once for the invitation to work.

Might try that thx

You’re a lifesaver Tysm