The vaccine is effective for several decades, and hep b can spread through bodily fluids not just needles. Exposure to another kid with hep b could spread to your brothers kid.
From what I've seen on social media from the soft anti vax crowd, pediatricians nowadays are pushing for a lot more vaccines compared to their parents' time. There is a reason for each individual vaccine, sure, but the difference is huge and something to be wary of.
Problem is the risk vs reward question seems to be ignored in favor of talking about just the reward part. I'm sure there are reasons for not talking about the risk part like you don't want to be too chummy with the anti vax crowd or you don't want to discourage vaccine use but it makes most people's bs detector go off. There is never reward without risk.
AFAIK it’s not spread through saliva, or maybe the probability of spread is just much lower? But yes, sloppy kisses from adult relatives are gross and spread other things like herpes.