Of course he could!
But why not try to prevent some of them, no I am not persuaded by any of this. Coming from a person saying "well they could die a different way soooo..."
WHAT?!
Why shouldn't we try to prevent as many risks as we can to literally STOP BABIES FROM DYING?
There's another perspective here which is that your baby brother is not going to get vaccinated against Covid any time soon (since the JCVI in the UK was ambivalent on vaccinating teenagers but ultimately decided against it for the time being, I don't see any chance of them approving vaccines for babies in the foreseeable future if ever), but Covid will become an endemic disease which everybody will be exposed to at some point. It will probably become a virus which most people catch in early childhood, perhaps several times, so that hopefully by the time they reach teenagerhood and adulthood they have a certain amount of immunity. Your baby brother has to get Covid at some point. It's next to impossible that he won't. Is it better for him to get it in early childhood when the risk of death is almost zero, or in adulthood when it's likely to be a more unpleasant illness with a higher risk of death? Ideally you would want neither but that's not realistic, just like it isn't realistic to expect him to get through childhood without getting chicken pox, flu, rotavirus etc. You can't shield him until he's 12 and if you did it would be worse for him than getting Covid.
We adults and older teenagers were unlucky in that Covid hit when we were old enough that it's a potentially serious disease for us and because it was new we didn't have any immunity at all. That's why vaccinations are needed, although I do think you need to work on empathy towards those who make different choices to you. You might not agree with that choice and you can say it's selfish all you want, but people have real worries and anxieties over vaccination and telling them they're being selfish is insensitive and unhelpful.
Remember that the online world tends to be a more extreme and polarised version of the real world. So you'll see the most extreme shades of opinion on both sides of the issue, from people who are saying that everyone who chooses not to be vaccinated is selfish and that's it, to those on the other side who believe vaccines are killing thousands and are all a conspiracy. If you spend a lot of time online it will influence how you view issues like this. You'll view them in black and white because that's how it looks online. Real life is made up of many shades of grey and you'll rarely meet someone with either of those extremes of opinion. You'll meet people with long term health conditions who are concerned about how the vaccine might affect their particular condition. You'll meet mothers of 12 year old boys who are concerned that the risk of their son getting myocarditis from the vaccine is several times higher than the risk of them being admitted to ICU. You'll meet young women who are concerned about the vaccine's effects of periods. There are many reasons why someone might be unwilling to get the vaccine right now and them being a selfish idiot is only one of them (and a rare one). I have people I know and love who are unvaccinated and none of them are selfish people and none of them get their medical advice from Facebook. Many really want to be vaccinated both for their own sake and others', but they have concerns that are still unresolved, and being told they're selfish by strangers online, does nothing to actually resolve those concerns. If you really want to understand a serious issue, don't limit yourself to the Internet, or at least social media which is the worst for polarisation.