Casbow - so sorry to read this attitude from your youngest.
My brother suffers from depression and this causes him to withdraw from difficult things, so dealing with clearing my mum's house (which he promised he'd do) and then selling it (which later on I asked him to do) didn't happen as they should. I ended up clearing the house (300 miles from me, 100 from my brother - I could only go for 24 hours at a time - he was meant to do it as mum came to live near me and he wasn't working). With my mum's house sale, I was emailing him to find out what was happening and he was just ignoring me as he hadn't done what he should.
It had a big impact on both me and my children, when at the time they needed me, as I couldn't deal with everything, and I can never forgive him for that, and when my mum dies I will cut all contact.
He seems so unkind. So couldn't care less. So sod you mum get on with it.
This is exactly how my brother comes across in total. I know he is ill but that isn't my fault, and ultimately he has always been very self centred and it isn't nice.
I do sympathise with you, there is little that you can do about it apart from encourage his wife to make sure he gets treatment.