When my stepmother and I would have problems, when I was very young, six or seven, when it got too much for her she would leave in the evening and go for a walk around the neighborhood. . . rather than kill me

Maybe when you get roped in to helping in family situations where you don't have much actual authority and it doesn't go well for you, you could deliberately get dramatic (ruins their criticisms if you do it on purpose), throw your hands up in the air and announce, "I give up!" and walk out of the house. Sometimes my husband will ask me to do something or my advice and I'll do/give it and then he'll start questioning it and I'll "give up" like that.

You are not responsible for what others do or how they perceive you; that's their problem. If you don't want to leave the house, try a bathroom and close the door in their face? Keep centered on how you feel and you can't go wrong because that is all you have to go on, whether you feel like you are helping, being heard, making a difference. If you are not feeling "loved" then don't play! It's not your battle, no matter how much you would like to help your mother.

Boundaries are individual affairs and tools rather than "set"/lines drawn in the sand or a barbed wire fence box around us. One tries to gauge the other's intents and reasonings and what can be "gained" by reminding the other person of who you feel yourself to be.

It does not sound like your uncle has ever "seen" you as a person so he's probably not going to hear you either. It sounds like he treated you as he would treat his own child, and you are a niece, a "child" in his eyes. It was great that you told him you did not appreciate his saying "shut up" to you, speaking to you in that way but you were not able to follow it through with any "action".

Boundaries are two-fold, the person states the boundary and gives a consequence if it is crossed again in the same or close/similar spot. Instead, you "shut up" like you were told to? You needed to assert your power over yourself/actions/voice by saying something like, "Do not speak to me like that again or I will leave". In this case, it doesn't sound like it would not make a very big difference, because your uncle did not want you there in the first place and the "power" was uneven because you were going up against an older generation. The only way I can see that you could probably help is if your mother were stronger asserting her rights and you were a male, lawyer nephew, instead of a niece. In this instance, you have no "authority" or anything other than an opinion to offer and your uncle doesn't care and doesn't have to care; you can't make another person care and, if you think about it from his perspective/family history, how can he suddenly listen to a niece? The family has never interacted that way to teach him how useful and right and thoughtful your opinion is and "worth" listening to?

I'm reminded of when I went camping in New Hampshire one summer and the campground we were at was having problems with a bear and they set a bear trap in the campsite across from ours. Another camper was telling us the story of his family's interactions with the bear a few nights earlier; they were sitting around the dinner table and his 4 year old son came and told him, "Daddy, there's a bear behind the car". He immediately thought, yeah, yeah, sure there is, and disregarded his son. His son told him again and he told his son to shush but his son kept insisting there was a bear so he got up and went to look and, sure enough, he turns the "corner" around the other side of the car and here's a female bear coming at him.

It was a funny story for adults to tell each other; how likely is there actually to be a bear in a public campground? How active are 4-year old imaginations and how inclined are one's young children to say funny/bizarre things? Your uncle has never "seen" you as an adult, you're a 4 year old and he's a bear; your mother has to do her own work or get a park ranger (lawyer) to do it for her?