@Crys Sorry for the trouble you are having with your Premium Family subscription.
I have created a ticket with our support team for you.
They will be contacting you privately via email to help look into this further, so please keep an eye out for their message.
Thanks for your patience in the meantime!
I figured it out! When you are going to Accept Invite. Click and HOLD until a pop up comes up. Click copy URL.
Go to your website of choice and paste. It will stay in website to ask you to log in, once you do the it will say your are now on family plan!