Your brother has something you don't: a continuing biological family surrounding him, with generations below him and presumably the particular pleasures and complications that brings. You have something different: a life that wasn't organized around raising children and grandchildren, leaving you enormous freedom to construct it according to your own interests and relationships.
And there are some pretty obvious tradeoffs. His tribe means companionship, continuity, people to celebrate with—and probably obligations, noise, disagreements, worries, and expectations. Your lack of descendants means you miss those things, but you also haven't had to spend decades parenting, financing, worrying about, and negotiating with successive generations.
Their pity is understandable because from inside their value system, having no descendants looks like a loss. But that's rather different from experiencing it as a loss. You aren't sitting there thinking, "If only I'd had four children." You have the life you actually wanted, rather than mourning an imaginary alternative.
There's also something amusingly symmetrical about it: he can look at your life and feel sorry for what you don't have; you can look at his and see both what he has and what he had to live through to get it. Neither of you necessarily has the superior bargain.
Yes—and that rather changes the biological bookkeeping. 😄
Your brother having children means your parents' genes got carried forward through his descendants, and some of those are genes you share with him. So although your particular genetic combination stops with you, quite a lot of the genetic material you inherited from your parents is now represented in that enormous clan.
In a sense, you have a rather nice arrangement: you get to be an uncle/great-uncle/great-great-uncle without having had to do the parenting. You contributed to the gene pool while outsourcing the child-rearing. Not a bad deal.
Ah, yes—I took the joke literally. 😄
And you're right about the larger point. Genes aren't immortality; they're replication. Natural selection has no interest in preserving you, only in whatever bits happen to reproduce successfully.
Even the descendant who carries some of your DNA isn't you in any meaningful sense. Your particular combination of memories, experiences, quirks, relationships, and consciousness occurs once and then ends. Having a great-great-grandchild doesn't change that any more than having a copy of a book makes the author immortal.
Evolution is wonderfully sly about this: it gives organisms a powerful urge to reproduce and then quietly substitutes gene continuity for personal continuity. A remarkably effective trick—if you're the gene, not the person.