I can understand a mother wanting to spend time alone with her grown up kids - to put another angle on this, my mil has a new partner and we never see her without him. I think that my dh would like to, sometimes (even though partner seems very nice, he is not family to my dh). But a week in NY is asking a bit much imo. The OP isn't a new partner, she is a wife and they have been together for a long time. It is a snub, and quite self indulgent of mil, imo.
Also, I know mil is paying for this trip, but it will cost the OP's family money. Who goes to NY and spends nothing? By the time you factor in meals out/drinks etc, it will end up costing quite a bit. And then there is the holiday time used up.
Like jacksmania said, the 'd'h has informed, not asked his wife. I'd expect to be consulted, not informed. When you have dc, they are 50% your responsibility and it's wrong to bugger off on a jolly and just assume that your partner will be taking care of them on their own, without even asking. That's just plain rude and taking your partner for granted.
OP, I'm not sure what you can do. I'd be very tempted to put my foot down and say no, on the grounds that I want my dh's holiday time to be used for us, his family and also that I would like to see NY with my husband, not have him go without me. When you get married your priorities should shift to your spouse and decisions should be made jointly, not one person doing what their mum wants and sod how the spouse feels about it.