We'd first recommend making sure that the family member you're trying to invite hasn't already joined your plan. You can double-check this here.
If they're not listed to your plan, we'd recommend sending a fresh invitation and asking your family member to accept it in an incognito or private window in their browser. You can find more info on how to resend an invitation here.
If that doesn't do the trick, let us know and we'll be happy to help. It'd be also very helpful if you could include a screenshot of what they see. Just make sure that it doesn't include any private/sensitive info.
We'll be keeping an eye out for your reply :)
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