One of the benefits of pushing 70 is that I have a first hand experience of the modern world; post WWI. I say WWI because when I became sentient, and began to be aware of my surroundings, at maybe age 10 or so, WWI's end was only 32 years in the past, a shorter time than we are from VietNam as a comparison. Schools taught American exceptionalism for the most part and because of that, we were the bastion of freedom in the world. On balance, we have been much more noble as a people and a nation than not, and we stand head and shoulders above the rest of the world in that regard, with few exceptions.
Having said that:
Your niece's worldly knowledge comes from being educated, probably, in a public school or a liberal private school where they have been trained up by left wing radicals from the 60's and 70's and their protege's. Her church, if she attends one, probably teaches liberal views that on the surface appears good, but upon closer scrutiny is not biblical except as it has been twisted much the same as leftists twist the Constitution. Her parents and the people she has been surrounded by probably don't have a world vision much better than her own. She has no idea of the reality of the last nearly 100 years except through the prism of the aforementioned "educators" and peers.
Unfortunately, you'll never get through to her or your other left wing family members because you don't have any credibility with them. You represent what their whole life experience, education wise, has taught them to disparage. The only way they change is when they go a bit further in life they get struck smack dab in the face with reality and they wake up. It's possible. But sometimes they never change, only get more dogmatic and continue to be like your earlier posting of Ronald Reagan's quote.
By the way, my dad and I were sitting in the living room, him on the couch, me on the floor leaning against the couch and watched TV as the Cuban Missile Crisis began to unfold...