We ALL want THE answer. Let me know WHEN you find it...
Wait a bit on the concert. If your sister will be understanding about the seat, maybe give it a couple of weeks to see how you and your H are doing. Otherwise, he might view it as pressure, and it might have a negative impact if he has to deal with that for weeks. Yikes, there's that word again... WAIT. Maybe, in the meantime, you can do something together with H and your S.
Relax. Appreciate. Be calm. Laugh. Enjoy. Be secure. Be loving. Be loved. Don't personalize. Don't ASSume. Accept. Be grateful.