Sometimes brothers don't know best. See, your brother is a language prescriptivist, which means he thinks that rules of language exist in some Platonic ideal, and that people must follow them for always, and that mutation, corruption and borrowing is wrong. Since language is actually, in fact, descriptivist --- the way people use language defines the "rules" --- he gets angry when English-speaking people say things like "novas," even though we don't speak Latin. The point of all this is that your brother is, similarly, also a musical prescriptivist. He thinks that dicta (I didn't write "dictums" to please Samurai Frog!) about music, like "punk is dead," can be asserted and that people need to fall in line with these "rules." But this is not the case. In fact, I find the very notion of making such pronouncements absurd.
Punk music is not dead. Neither is baroque, disco, grunge, or polka. These forms have passed their prime --- they're no longer as popular as they once were --- but someone, somewhere, is playing these kinds of music. And that makes them very much alive. "But," your brother or some other musical prescriptivist might say, "punk and grunge, and to a lesser extent polka, are ideas and cultures, not just music." True. And someone, somewhere, is living that life, embracing that culture. Music doesn't "die." It especially doesn't die at some set year just because some cranky old fart thinks that was the year the last "really good" record came out. Hell, I have a CD of Peruvian flute music. It's a living, breathing scene, man!
Now, my favorite band of all time is the Clash. Your brother is right about them. I own every single thing they recorded, and some of Joe Strummer's solo stuff, and even, God help me, that horrible spin-off band Big Audio Dynamite. I also dig the Sex Pistols, the Heartbreakers, and the Ramones. (I always found X kind of boring, though.) Am I a punk "purist"? Hell no! Punk isn't about saying what is or isn't cool, or what you can or can't listen to. Two of my other favorite bands are Rancid and Green Day (and I'm mystified as to why ol' Frog says the latter started out as ska), bands that have been accused of "stealing" punk, "selling out," or any other bit of nonsense you can name. I say these bands are punk, and they're great. Yes, Green Day has a lot of pop songs as well, but seriously, so fucking what? The Clash recorded songs of every musical genre under the sun. The Ramones recorded "I Wanna Be Your Boyfriend," a schmaltzy ballad explicitly intended to cull in the female demographic if ever there was one (and a great tune, to boot). The only difference between Green Day's "Time Of Your Life" (a terrific kiss-off song) and the Buzzcocks' "What Do I Get?" --- also a poppy song --- is airplay. Believe me, the Buzzcocks would have loved to have a major hit. Too bad they sucked ass!
These modern bands that your brother says are bad --- maybe they are bad. But maybe you enjoy them, and why the hell not? Jesus, don't let people dictate to you what you "should" be listening to. For God's sake, that's the polar opposite of everything that is punk! Personally, I think Franz Ferdinad is the epitome of overblown, shitty cock rock ("I'm gonna make somebody love me, and you're so lucky?" --- shut the fuck up!), but I wouldn't for an instant try to tell someone who enjoyed them that they weren't punk or cool or some shit.
Some negative people just like to sneer when something isn't up to their own particular standards, and they justify it by dictating what's cool or, in this case, what's punk. I urge you to read this very intelligent diatribe by an author named David Eggers against the dangers of falling into this negativity trap. The gist is, and I quote, "What kind of small-hearted person wants an artist to adhere to a set of rules, to stay forever within a narrow envelope which we've created for them?" But really, read it all. Several times.
Bottom line. Your tastes are your own. No record company, chart position, or fad should sway them. Be your own person. Decide what you like and what you don't like. Don't let people tell you what's cool and what's not. In short... be like your brother. I'm telling you, he's pretty cool.