Acceptance of your membership request is dependent on the Site Manager of the site you requested membership for.
If that person hasn't visited his/her family site or happens to be on vacation, they will not answer you right away.
Be patient and give the Site Manager time to respond.
As an alternative way to get in touch with the Site Manager of the site you'd like to join to, you can write a comment on the Site Manager's profile page.
Explain your possible connection and the reason for your request of membership.
The Site Manager will receive an email that you left the comment which should help to get their attention.