Probably achievement.
Talking typical manga. Girls are usually the achievers. They have a target in mind and work to improve themselves for it. Be it academic, social, or keeping their looks (after all, youth is limited and your clock is ticking). They also want to be as perfect as possible to protect themselves, so the society can't find excuse to talk bad about them. In other words, they choose build up their weapons to fight a war rather than running away.
While the boys, at least the protagonists, typically got no target in life, felt they can't keep up with the pressure for achieving against the competition who already got head start, and then ends up running away by avoiding everything by running to the safe zone of being a loner (you won't offend anyone if you don't interact with people, you don't have to feel disappointed if you never have any expectation, you don't have to get embarrassed when making failure if you don't do it). It's less tiring than how the girls do it.
Shizuku seems to be that kind. Good looks, polite manners, good in home skills, and I assume she probably will be going to a better university than her brother.
So, normally her parents would be hoping more and try to support her. Compared to Kouta who despite at that age already, has not shown any result nor motivation to improve himself.
Not saying he must become rich or work traditional job. If he could at least clean his look, show he got "good people" friends and able to be "part of the society", it would've been more possible to convince the parents his job as porn manga artist is something he can be proud of without damaging his ability to be part of society.