I had a family account and can send invitations to new members. But when I try to send it to my wife's current spotify accound which is free at the moment she can not join with same adress other new members join.
This is odd. As long as she lives at your same address, she shouldn't have a problem to join your Premium for Family plan. Can you send her a new invite? We suggest that she tries to accept it from a computer, using a Google Chrome incognito window.
Make sure that she logs into the right account when accepting the invite.