Can’t you talk to your brother and induce him to either rearrange his idea about DM visiting ( eg in six months’ time, maybe) OR ( and preferably) pay for you and your mum to spend a week away somewhere nearer home, which offers plenty of opportunity for you to be alone with your thoughts, in an inspiring location? Your mum needs a change of scene, and you'd still be there with her even if you happened to be off exploring ruined abbeys solo at the time.
If you don’t address this with DB, and you remain understandably adamant about not going, there could develop a nasty old ganglion among the three of you.
Your situation after your father’s death ( and my condolences on this, of course) is similar to mine over fifty years ago, though there was no husband adding to the misery.
Ma lived abroad (not far from GB though) and I was visiting, and Dad died suddenly. I sorted everything out locally of course, my brother and other relatives just coming over for the funeral.
Then Ma and I drifted into an easy companionship (which we’d never had before), also taking little trips around the county, though in the end we upped sticks and came back to Blighty.
But ever after, and especially in her old age, no matter what misery I brought to her door, whatever unkindness she received from others’ hands, she always held on to my having stayed with her when Dad died. She treasured the fact that I didn't leave her.
She saw that as a supreme kindness, God love her, whereas I didnt know what else to do with myself anyway.
But it still comforts me, long after she died, that she attached that fond memory to the devastation Dad’s premature death wrought.
I mention all this because your mother might be using your presence as a sort of ‘fireguard’ right now, as she processes her loss.
If you were to be freed from the prospect of this trip to your brother’s place, could you use a different, much lower pressure trip to make plans about your own future? All the while still being around for your mother?
I’m very sorry for what you are enduring, with trouble howling at you from many different places. I do hope you can break it all into smaller, manageable pieces, and find some serenity.