Dear Mealybug, I am so sorry to know what you & your beloved are going through at present. It's unimaginable to those of us who have never encountered such circumstances but I hope you feel Gransnet opinions can be helpful and that you have other loving support at this distressing time.
When my dad passed away many years ago, he wasn't exactly estranged from his siblings but they'd shown little interest in him during various incarcerations in hospital. I therefore didn't bother to tell them until after the cremation had taken place. Only one cousin still brings it up occasionally but I just change the subject. It has not caused estrangement in the wider family at all.
I suppose you have to ask yourself how bothered will you be if you never see or hear from the brother again if you don't tell him of your Dear One's demise? It doesn't sound as if they will bother with you if they can't be in touch with your Beloved in his hour of need so what have you got to lose? On the other hand I suppose you could let him know but be very very circumspect with him at the funeral. If it sadly transpires in the time of Covid I doubt you'll have a wake (?) so just give him a wide berth & monosyllabic if he speaks to you. He'll get the message.
I hope you'll find a way forward which will emotionally sit easy with you.
Estranged Son and Future Granddaughter
To think that London, or anywhere else for that matter, does not belong to any one demographic