Elderly mother is taking care of Elderly sister/aunt — Aunt’s kids are not stepping up

My 70 year old mother, who is dealing with her own health issues (auto immune disorder) is the main person taking care of her sister (my 74 year old Aunt) as they both live in the same Arizona town. My Aunt’s son, 54 years old, lives in Montana. He does not help out with his mother at all, though he is a single man with few obligations/responsibilities. My mom does all the work and I am worried about her own quality of life. She is not the type to stand up and tell my cousin that he needs to step up.
I see myself judging my cousin (the son) and I am tempted to write him a letter expressing my concern. I don’t know that anyone else has spoken up to him about my mother’s health issues and I want to make sure he is aware of the physical strain this situation is putting on my ailing mother. Stepping back, I see myself thinking about all this and I do realize that for me, this is all a circumstance. I have no control over what my mother chooses to put up with, no control over what my cousin chooses to do or not do, and I can’t expect his behavior to change by requesting that it change. Likewise, I recognize that it’s my mother’s choice to take care of her sister as well.
So it makes me wonder… should I even write a letter? If it won’t change the circumstance, does that mean that I shouldn’t write to him? I think it would make me feel better, as if I did something about it, but I’m not sure that’s a good enough reason. “Make sure you really like your reason” Shoot.
Is there any value to doing this for myself, or is this just negative thinking in disguise (observe myself thinking “I wish Michael would take care of his own mother.”)