Looking at my friends, some I can see maturing and some I cannot. It depends on the level of immaturity. I'd be willing to bet that your brother is NOT going to change his ways.
I have one friend, he's mid-30's, and he's an idiot. He's terrible with money, sees credit cards as free money, gets $100,000 into debt then his parents bail him out, he is at bars and clubs every Friday and Saturday, he cheats, he's been in a weird off/on relationship for a couple years, never married, no kids, buys a new car every year, is an idiot in relationships, etc, etc. There is NO chance in hell this guy is changing if/when he gets married and I frequently feel bad for whoever he ends up with - though, she'd be an idiot too.
I have another friend, he's late-30's. He has never been married and does not have kids. He didn't even lose his virginity until a year ago. We were all SURE he was gay but he said he was waiting for "the one". Whatever. He has a stable job, owns a house, seems to be good with money, but he's a idiot when it comes to women and relationships. He says dumb stuff, expects dumb stuff. He's selfish, he can't apologize when he's wrong, always has to be right, he was going to bars and clubs every weekend (no idea why), he's needy and clingy, etc. He's in a relationship now, it seems to be going well and he's slowly changing. Everyone is noticing it.
Another guy, mid-30's, just had his first baby. He didn't mature when he got married and sure hasn't matured since having a baby. He spends no time with his family, doesn't help, doesn't clean or cook, and instead spends 8+ hours a day playing video games. He was doing that before marriage, before kids, and he's still doing it after marriage and kids. His wife was a fool for expecting him to magically change.
And, I was an immature ******* until I was abruptly forced out of it.