Congratulations on your new additions! By all means, stay at home. Disney is very crowded during that week, and it's overwhelming for kids who are older than your two. Children do not understand crowds, they don't understand waiting in line, they don't understand the sensory overload of the parks. They are too young for most of the rides, and they don't even know who most of the characters are. Just look at all the kids crying because Pluto or Goofy came up to them! I love Disney but we never took little kids there.
It's not purely a financial decision, although it is a very expensive trip. That doesn't mean you have to overwhelm your daughters with a lot of Christmas gifts either. They are going through a major adjustment, so keeping things calm while you get established as a family is a good thing.
I don't think you have to make the decision right now if you aren't ready. What's the point? Do you have to face all the family members and justify your decision? You've already introduced your daughters to this big family, is that right? So you're already being pressured to sign up for more of this? I imagine there is some desire to make reservations but otherwise what's the hurry? Go home, calm down, get the girls reoriented to your household, and take a breath.
If your wife is on the fence, she's not totally committed to Disney anyway. She may just want you to make the decision so she doesn't have to be the one to confront her family and their pressure. That kind of makes you the "bad cop" which may not be pressure you want to bear on your own. But if she's not committed, and you're not committed, that's your answer.
You should also take this time to evaluate your family finances. Two children means you need to start banking money for all their expenses, not just the routine stuff but the braces and the swing set and the activities fees and the college tuition. Maybe a high-end trip every year is not going to work for you. You don't have to decide all of that now, but just decide that maybe 2014 isn't going to be your year. It sounds like none of the cousins get to spend Christmas at home either - that's kind of a shame, but I digress. FYI That 15 year old is going to get tired of this Disney routine soon anyway, and his/her parents are going to be socked with college costs in a couple of years, so the whole thing may disintegrate unless everyone is fairly well off.
Do what you need to do. Traveling on the holidays is very stressful for many people, and by September Mamapedia posts are going to be filed with laments about having to split holidays with 2 sets of in-laws or shuttling kids to 2 Thanksgivings and never being at home.