I’m also not sure I agree with your assumption that a 6 month older girl would be more “developed” than a boy. Social and verbally, that’s a good guess. But probably not physically, in terms of gross-motor skills. That is, I’ll take your word for it that this particular girl is more developed, but it’s not a blanket assumption. Boys tend to be a bit ahead of girls at that age. Statistically speaking, I’d have guessed they’d be about even, but there’s a wide range of ability at this age.
Anyhow, I see several issues here:
ONE, is it good for a kid to be the follower? Sure, why not? Some people are leaders (as it sounds this girl is) and some are worker bees. Absolutely nothing wrong with that, and it’s something that will change a dozen times as they grow. If she’s just started kindergarten this fall, she’s probably more exposed to “bossiness” at school and wants to try out that role. This too shall pass.
TWO, is it good for a kid to ALWAYS be the follower? No, probably not. But, in this relationship, she may be the alpha. He should be encouraged to play with other kids other days when he can try on the leader role.
THREE, should kids be paired up or grouped according to age and/or development? To this I offer a resounding NO! Kids learn from each other at least as much as they do from us, and it’s best done in a mixed age environment. If you’re always with people who know and do what you know and do, any innovation comes only from serendipity, and that’s a good way to get stagnant. I despise age grouping in daycare and school. That’s not to say they should never play with age-mates, they should, but they should also play with kids and adults of other ages, abilities, interests and backgrounds.
FOUR, the question in the OP, is your brother pushing your nephew too hard. “Too hard” is hard to define, impossible to identify in one day, and I don’t see evidence of it in the OP. Some kids need to have their boundaries pushed, because they won’t do it themselves. How do you know when it’s too much? When the kid shows a change in personality or coping skills over a period of time. If an outgoing kid becomes shy, or a problem-solver becomes helpless, or a generous kid becomes selfish, these are signs that the parents are trying to make the kid into something they’re not. An isolated meltdown from tiredness isn’t neglect or abuse. It may be a temporarily flaky parent who was having too much fun and didn’t judge the kid’s capabilities in time. If it’s a pattern, then I might be concerned. If the parent isn’t sympathetic to the kid, (if he yells at him not to be tired, or tells him he’s being a wimp or something) then I might be worried. If he laughs at him, I’d be worried. Maybe some of this happened, and that’s what got you alarmed?