I also saw it as a jab; and an untrue jab at that. Military families all function in a "changeable world." They have no choice.
And not every member of a military family *chooses* to be a member of a military family. You can be born into one, your child/sibling can decide to join up. Certainly, some people chose to marry a military person, but the majority of people in military families didn't have a say in the matter. They just cope as best they can, just the way everyone does. It's just that different people cope with things in different ways.
Change happens a lot in military families. As a child with a military parent, I knew we could move with less than a month's notice, in the middle of the school year, to a country I knew nothing about. Absolutely nothing you can do but cope. Three months ago, my brother was told, after a year of suspense and back-and-forthing, that he was not going to Afghanistan. Guess where he is today? Hint: He's not in the US. My other brother was supposed to retire from the service this past August. Guess what? He's not retired yet, and can't get a firm date on when that might happen. And we were all excited that he might be in the US for Christmas this year and we could get the whole family together, which hasn't happened in over seven years.
You learn to cope with this sort of thing, but I've spent my entire life with at least one family member in the military and sometimes you get tired of coping and smiling when you don't feel like it and trying to make the best of things.
It would be nice for the OP's parents to get a happy surprise, if her brother gets leave, instead of a sad surprise if he tells them he's coming home and then he can't.