I don't have any words of wisdom for you, but I do have a little story from my own family to share - it may give you a different perspective.
My aunt (my mom's favorite sister and my brother's and my Godmother) has been diagnosed with Alzheimer's. She's 60. She was diagnosed in February (after having been tested several times over the past 10 years because of symptoms), but she didn't tell anyone until April, because she didn't want to upset my mom or me when I was getting married, etc. After she told my mom about the diagnosis, my mom cried and cried to me, etc., and then finally spoke with my aunt's husband. He's a jerk too - always right, always a major pain in the a$$, etc. He told my mom that he's thinking of moving back home (about 5 hours north of where they live now) to be close to my mom and their other sisters. The reason he gave her is that when the time comes that my aunt can't function on her own, he doesn't want my mom or their other sisters to take over because they don't think he is capable of taking care of her. He knows that he will certainly need help, but he doesn't want anyone to take the decisions and day-to-day stuff away from him.
Now we all know that many men are useless when it comes to caring for a very sick loved one. And we also know that men will ignore the state of being of a sick loved one. So when an aunt or a mother or a sister gets sick, our female instinct is to come to the rescue, and take over her care, even if her husband is right there. We see it as helping; he sees it as a slap in the face.
Maybe your uncle is feeling really powerless, and kind of shoved out of the way. And maybe that's why he's being suck a jerk to your mom. He's probably in a bit of denial about your aunt's condition, and he's probably grieving and frustrated and sad and angry, and worst of all, can't do anything to help her. I'm not saying this gives him the right to be a jerk to you or your mom or anyone else, but I wonder if it might help you understand what he might be going through.
I don't know what you can say to change anything or to make him feel better. But I do know that when my dad is down or seems to be feeling left out, I will buy a greeting card and put it in the mail to him (sometimes send it to his office so it's just between me and him and my mom doesn't know anything about it). He really may just need a shoulder or a cheerful word.
Just try to be patient.
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