I think it would have been totally fine to say to your Mom that you wanted a few minutes alone with your Uncle. Then talked to him after. Gee, it isn't about her.
I am really sorry that this is how you will remember that time with him. Is he close enough by that you could check in on him again?
Is does sound as if no one ever set boundaries with your Mom and let her do whatever. I know I am one to talk...my dad could get kind of scary. I am just now able to find my voice and more so when it affects my kids.
Thinking of you and your family.
kat
Me-53(and learning!) S24, S21, D18, D17 Just keep swimming, Just keep swimming. Dory