I have to chuckle at this---really, your mom said exactly what I was thinking. My brother is like yours...very "involved" on the surface, but if you really look at the amount of time or tasks he does perform for her, well....it's really very little. I've accepted that this is what life holds for me right now (taking care of her) and that I will have the rest of my life to go wherever I want to go and this is the right thing to do. He, on the other hand, has not had any change in his life at all. Unless I call him (he lives about 20 mins away and is in our area all the time for work), he wouldn't call her for weeks at a time.
He actually had the nerve to gripe at me a week ago because "the relationship" my mom had with his son (grandma/grandson) is really not there anymore.

Gee, sorry, she's not interested unless you work at getting her interest! Sorry she has dementia!