I appreciated your last post, much better than the original one. I'll just share some things from a father's point of view. Based on everything you wrote, you sound like a great kid, your parents are probably more proud of you than they are willing to admit.
1. I read a book once that said that inside each of us is a parent, an adult, and a child. All interaction is either as parent-child, child-parent, or (hopefully) adult-adult. I have seen plenty of 40 year olds who still can't interact with their parents at the adult-adult level. As parents, we get so used to acting as parents it becomes very difficult to allow our children to interface with us as adults, and it's equally difficult for teens to get out of the "me first" child viewpoint and to interface as an adult.
2. One of the prime responsibilities of parents is to try to put their children in situations that increase the likelihood of success, especially where the possible downside is so serious as is the case with driving. My boys were all good drivers, and I allowed them to learn while driving on family trips, but they were not allowed to drive anywhere with more than 1 friend in the car until they were 19. I just didn't like the odds that having more kids in the car created. Other fathers have a lot more trouble in unhooking their "parent responsibilities" and letting their kids drive for the family. Driving isn't even close to working on catching fly balls, but you are right that it would be benificial if your parents could find a way to help you get more practice. Ask you dad if you could do more driving on any longer trips to help you get more experience.
3. Your ages are 16,15,15,??. Your parents are looking at several years of having multiple kids in college. It wouldn't surprise me if they are seeing this as their last vacation for the next 7 years. That's the financial responsible side of the issue; I doubt that they realize yet the emotional effect of having the birds leave the next so close to each other. They may, in fact, be over-reacting a little bit.
4. I was glad to hear a subtle shift in your tone. Skiing this year seems to have declined from something that you deserved to just something that you would really enjoy doing. You might mention to your mom/dad that you will really miss the family ski days, especially since this might be your last year with the family (while indicating that you understand why the decision was made.) You may, in fact, owe them an apology depending on how you reacted to their vacation announcement.
5. Suggest that, at your ages, your brothers and you would be just as happy with a couple more days of skiing anda a little bit less for Christmas.
6. Be honest with your mom about appreciating that she has done the "family thing" all these years for the sake of the family. Suggest that you (and your brothers) are old enough that she doesn't need to feel "obligated" to come along any more. It's can be real hard for a mother to deal with the change of still being a mother when the children don't need as much "mothering" (in the sense of taking care of the kids). It's hard to let go. Some of her reaction may be because she didn't feel as "needed" last year, which made it even harder for her.
7. The hardest thing in the world is seeing events from the point of view of others. I would almost bet that your parents are seeing a multi-year picture and not just skiing vs. Martha's Vineyard this year. There may even be other financial considerations that you aren't aware of.
8. I would be willing to bet that you will wind up skiing 10 days this year, regardless of the state of affairs as you see it now. By the way, your ski day average is skewed by last years numbers. Another view is that several times your family skied quite a few days one year followed by far fewer days the next year.
Best wishes. By the time you are 25, the number of ski days when you were 16 will be insignificant, no matter how big it seems now.