Brother flitting between households

[Putyourdamnshoeson]

DB 32, in ordinary times he lives with DPs in the week and his GF at weekends.
I have found out, through social media, that he is still doing this. I raised it with DPs on phone (we live a plane ride apart, but still all in UK) DF mutters to DM off speakerphone and she says loudly, to him that it's none of my business. We don't have a good relationship, I do with DF not DM or DB, who often gang up on me (and other people, it's their thing).
DM is early 60s, and vulnerable DF a few years older and technically not vulnerable but overweight. Both smokers. DB is type 1 diabetic, sent home from work 3 weeks ago to shield and about to start a complex wfh set up today.
DB's GF works at a GP surgery.
I'm furious about the shuttling back and forth, presumably upping the infection risk significantly.
AIBU to worry? AIBU to be cross?
I know IABU to chickenout of a further confrontation on it. I am a professional 40 Yr old woman, who raises delicate matters sensitively for a living fgs, and yet I just don't know what to do here.

I'm pretty sure they know the theory. That's why I've concluded that I've got to leave them to it. But still feel cross

But the three of them have all agreed that this is how they want to behave. They are all adults, albeit stupid ones. You have to leave them to it.

And no, definitely don't disrupt your life to look after any of them if they get ill.

You don't get on with them, and you live a flight away?! I'd say you need to butt the fuck out!!

Ah, thank you so much for your Co siderd and nuanced response @Mistystar99
It's almost as if things are back and white and that families don't have quirks and alliances.