Our mother treats my 30-year-old brother like a child

My elder brother, an exceptionally bright individual, has excelled academically throughout his life, from elementary school to university graduation, setting a high benchmark that I've often been measured against by our teachers.

He currently holds a well-paying job with a monthly income exceeding VND50 million (US$2,038), though a confidential source, who is a friend of mine and manages his taxes, suggests his actual earnings are even higher.

Despite his financial success, my brother prefers solitude, rarely engaging with people outside his immediate circle and showing little interest in family engagements. His interactions are primarily with our younger cousins, who are attracted by his computer.

His after-work hours are consumed by side projects or video games, maintaining a reclusive lifestyle that began in childhood. This isolation extends to family gatherings, from which he is conspicuously absent.

With the recent Tet Lunar New Year, my brother's lack of participation in preparations was evident. He chose to spend evenings out with his girlfriend, returning to immerse himself in his computer and often sleeping until well into the next day. His only contributions were purchasing house decorations and suggesting we hire a cleaner—offering to cover the cost—because he believed that doing cleaning tasks during Tet would make the holiday "very tiring."

His avoidance of social duties at a family memorial ceremony during Tet, which our extended family attended, led to perceptions of disrespect, a view he dismissed when I confronted him about it.

When I expressed concerns about his behavior to our mother, she defended him, citing his work-induced fatigue and the need for rest during holidays.

Also, whenever we discuss family finances, he proposes equal contributions despite his significantly higher salary, though he's willing to cover expenses if needed, which I think is immature.

My mother's indulgence of him, treating him as though he were still a child, frustrates me. I believe this pampering would extend to any of us under similar circumstances, however, I've directly addressed his over-pampered status with him, only to be met with dismissive remarks about his youth and attractiveness.

Yet, he displays a contrasting, idealized masculinity in front of his girlfriend whenever she visits our family. They have plans to marry within a few years, delayed by financial goals, so I seek advice on guiding him towards maturity and greater family responsibility so that he can take care of our mother and his own family after marriage.

What steps should I take now?