OP You keep going on about fairness, but it might help to try thinking about things this way, and avoid bankrupting yourself.
The following may NOT be what happened, but, just as an example, consider this:
Your mother knows she is dying. Like many older people she does not want to die in hospital. She's frightened and in pain. She also knows that care home fees are extremely expensive : maybe £1500 per week or more. So is getting care-at-home. That often also involves extra expenses, such as heating, laundry, food...
Just perhaps - we don't know - your brother suggested that your mother live with him. How long was she there - two-three months, I think you said? That saved the estate (say) £25,000 .
The following are just notional figures, but say your mother left £100,000.
It would surely be "fair" if that sort of sum saved by home care went to your brother, in addition to his "equal " share. As other posters have said, it is no joke looking after a terminally ill person, even for someone like your brother's partner with nursing training.
Under an arrangement like this, two of you siblings would get (say) £25,000 each, while he would get £50,000 - plus, of course, whatever else your mother chose to leave him. She might have felt so relieved and grateful to be dying in family surroundings that she left him a very generous additional sum.
I'm not saying that this happened. I'm just saying that it is just one possible "innocent" scenario to account for your mother's will. And it's not an "unfair" one.