You know... I wonder if this situation wouldn't have gone better with a little white lie. Just to smooth things over and because sometimes it is better, especially in time of grief, to sugar coat things. It was a bit of time after his mother died, but it's his mother. I'd give a good amount of time with sugar coating things, at least a year but more depending on the illness or closeness.
OP, for your questions, I think your mom was a bit in the wrong. If she wanted to make sure that your uncle got the money, she could have sent it certified mail. If she did want to make a nice gesture, then she needed to be prepared to follow up what she wrote with the white lie of "yes, she did". There were probably still a lot of wounds when the thank you note came up. I do have a question - did he ever send a thank you note for the wedding present? The real present, not the check of his inheritance.