Op, another thought, forget family, are many of their friends married? In my experience, the 20-something general "yeah, marriage and kids, want that at some point in the future" isn't all that focused until one couple goes first. It always tends to go in waves, and wedding invites do lead couples who are happily bobbing along as they are, to have those "grown up" conversations.
At 23 and 26, there's a good chance some of their friends are only just finishing education, starting out in work with debts to clear from that before starting to think about saving up for rings and wedding costs. If none of their friends are going through the wedding planning stage, might just not be on their radar as something to do right now, more at some point in the next 5 or so years.
Or more sadly, it could well be while your brother cares for his gf, he doesn't see her as someone he's going to marry, that marriage and kids are what he wants, just maybe not with her- now he might never admit it, perhaps not even to himself, but men I've seen dragging their feet are often just doing so to avoid having to face up to that. (A mortgage is a commitment to a bank, not the person you took it out with).