Bea, so sorry this has happened to you.
I would write a letter to her, knowing it won't receive an answer, telling her everything from your point of view. How your brother was bullied and wanted only some contact with his dad, how you were going to meet your dad at the hospital, how you felt when you bumped into him at the supermarket, how wonderful your children are. Get it out and down on paper.
Then burn it.
My dh's dad is strangely estranged; there are occasional phone calls, birthday or xmas cards intermittently, expressed desires to meet his gd - his only gc - but they all come to nothing. Dh is hurt over and over again. Now dd is too. I think dh has found the strength to cut his dad off because dd has been made so hopeful and come crashing down. Dh's dad is just a weak and selfish man, who suffers from the odd prick of conscience which makes him phone or write every few years, raising everyone's hopes; he's too cowardly to actually see dh, and now dd too, face to face.
I think your dad was similar. When confronted with his responsibilities he plays along until he can run away, which he does very fast, simply by not turning up. His mother does the same but her way of evading it is to be mean and hard.
Get it out of your system and then forget the pair of them. Mind you, nothing wrong with checkingmthe will - might as well see if you can get something from the old bugger, think of it as compensation. 