Thanks for reaching out and welcome to the Community! We'll happily help you sort this out.
Keep in mind that your brother's account must match the country setting of the Premium Family manager in order for the address verification to work. Since he created his account in the UK it's possible that the account is still set to that country.
He can change his country settings by following the instructions here. If that doesn't fix it, we suggest he tries to accept the invite using a private or incognito browser. This step ensures no cache issues will prevent accepting the invite.
Hope you find this useful. Keep us posted how you get on.
Have a great day!
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