Dog in the manger

How is it that person A can feel miserable because person B is doing something that person A didn't want to do in the first place?

And that person A's family feel guilty about it. To the point where it spoils the event for them...

Nothing more to say really. Can't stop person A being ill or jealous or depressed; no point in reasoning about it. Can't exclude person B, wouldn't be fair anyway. Just wish the situation had never occurred.