My kids (ages 4 & 6) drink mostly water; if milk is on sale, I will get that and let them have one cup apiece per day (maybe more if we make it into hot chocolate, but I'm not big on cow's milk for human children); they also usually get one cup of juice per day, which I'd like to get them off of because I don't like that it's so much sugar, even if it is "natural" sugar. Occasionally (at parties, church lunches and functions, weddings, the rare long trip where we get fast-food on the way, things like that), they'll get lemonade, caffeine-free soft drinks or Kool-Aid.
It may be tough to transition them off flavored sugar-water (which is what most beverages are, if you look at it that way), if they're used to drinking it, but I think they should be encouraged to drink water. It's just not a healthy life-habit to always drink flavored stuff, especially when there are concerns about artificial sweeteners (I have those concerns too!), and sugar just contributes to the obesity epidemic.
In regards to the differences in the two boys (I'm assuming they're biological brothers, and not that you just happen to be adopting two unrelated boys at the same time), I wouldn't worry too much about it, although if you're worried, you can bring it up at the next doctor's visit. I've known many sets of siblings, both boys and girls, in which the younger child outgrew the older one. [In fact, I rather suspect that my boys may be doing the same in a few years. They're 19 months apart in age, and the younger one has consistently outgrown clothes that are "his size" for his age before the older one has outgrown clothes that are *his* size. My 4-y/o is outgrowing his size 4s, but my 6-y/o can still wear many of his size 5s, and some of the size 6s are too big for him.] It may make the sibling rivalry that much worse, as the older one tries to retain his "natural" higher rank due to his age, while the younger one seeks to take that rank due to his size, but I don't know that it's a health concern.