I think these precautions are fine. And I actually don't even think you need the health concerns to justify them, though I understand those concerns. You have a baby who is a week or two old. You want to protect the baby and limit interaction with other people. This is not only normal, it is likely a biological imperative. These are perfectly normal instincts to give into for a child under 6-8 weeks of age.
In many cultures, it is not atypical for new babies to be secluded with their mother and maybe a handful of close family members for the first 6 weeks of life.
Consider sharing a resource like this with your DH:
https://www.hopkinsmedicine.org/health/wellness-and-prevention/new-parents-and-newborns-are-visitors-ok
It's a practical guide for making these choices that emphasizes that newborns have immunity vulnerabilities and that it's okay for parents to set boundaries and limitations during these first few weeks. For instance, it is clear that the recommendation from healthcare professionals would be to keep your baby home from the holiday performance, which would be a "large crowd" -- there is simply no good reason to bring a newborn to something like that.
I would also emphasize with your DH, that your baby is X DAYS old. Like frame it that way. Say "when your brother and his family come to visit, the baby will be only 17 days old." Emphasize the difference between being in this stage, where you count the baby's lifespan in days to a 6 or 7 month old baby, as your older child was for his first holidays.
I fully support you in standing firm on this. I didn't stand firm on some of my boundaries during this postpartum period and later I felt like I'd been bullied into doing things I wasn't comfortable with.