Big Congratulations!!!
I remember being in exactly the same situation, every time Maddi would wake Keira would suddenly wake and want to help too - great

I guess all being in the same room didnt help.
I cant really think of any words of advice, except the novelty quickly wears off for the older one and it before you know it they are totally over it and are back to sleeping through anything.
Grr at DH, I would be giving him a big kick and getting him to help!