Update.
I decided to contact Apple via online chat. I explained the situation and sent screenshots to make sure the agent really knew what is happening.
I don't think he had a clue to be honest but then maybe he did. After a lot of pauses and "checking", he said to try clicking cancel because it won't affect the membership status. Clicking that produces a pop-up.
Cannot Complete Action
Unable to remove invitation
OK
So that was no use. The invitation won't resend or cancel. It is just stuck.
After another pause, he came back with (copy/paste):
"A pending Family Sharing invitation is shown on your device yet you're unable to remove / delete the pending invitation!
You might or might not receive an error when trying to remove / delete the pending invite!
The best option it's try waiting for about 15 days after the invite was sent, at which point the pending invite should be automatically deleted!"
(Apple agents love an exclamation mark.)
I was invited to chat again if it is still there after another 48 hours. My crystal ball, although not always reliable, says nothing will change. It has already been 16 days so the 15 he said that were needed have already passed. Still, you have to do as asked. I hope he is right but we'll see.