I don't think this would be on. You would be going from one area to another and back. and going to see the care home, and being in contact with your brother and your mother - I don't really think you would find it possible to travel all that distance without seeing your mother, even if she doesn't recognise you. Also, you are helping out with grandchildren, so in contact with another generous source of infection.
Many of us are in the same boat, unable to see elderly parents for their sake and for the sake of the whole community. Can your brother arrange it all himself, keeping you informed at every stage? You can have discussions about every point he has questions about.