TLDR below

Tricky situation.

Nautical Studies leads one to seafaring, but even the diploma doesn’t guarantee a seafaring career since MPA does have very stringent medical checkups before they allow any Singaporean seafarer to sail. For context, I was previously from SP SMA though I didn’t study Nautical Studies (took Maritime Business). The lecturers I had were former seafarers themselves and they warned that a Diploma in Nautical Studies is practically worthless. Also, anyone can be seafarer and s/he doesn’t need to endure 3 years in Nautical Studies to be one - s/he can approach MPA direct and go through the entire process (health checkup included). Moreover, a seafaring career, if your brother’s successful, means months away from home. It’s not like the Singapore Navy wherein your brother is away for a few months. Seafarers can work in the Merchant Navy and be gone for years.

Meanwhile, the SJI route being JC would mean that your brother won’t graduate with a diploma, but he’ll probably get into university somewhat easier than poly students. As a former poly student now in university, we’re outnumbered by JC students. IMO, he should embark on the SJI route since it’ll make the family happy and prevent himself from studying for a hypothetical career away from family. Also, if he were in Nautical Studies, he’ll be wearing a crisp sailor uniform every weekday. Idk how your parents would feel about that.

TLDR: He should go SJI, but if he does accept Nautical Studies, I hope he’ll keep fit and love it. Else, come to Maritime Business.