Have you asked your sister to explain to you why she believes the earth is flat, @Astro, but listened with the intent to learn? I’ve watched a number of videos of people explaining why they think the earth is flat, what information they put forward to back up their claim, etc., and some of them were extremely interesting; and, while I personally wasn’t convinced, I did think that some of the arguments were put forward very well to such a point that I could understand how some people might decide that the “evidence” to the contrary could be legitimate. So, I guess it depends on whether or not you genuinely want to understand where your sister is coming from, or if it’s better for you to be able to mock and proclaim her idiocy.
To “understand” doesn’t mean to “accept”, even though people tend to conflate the two. Maybe with your sister it’s completely emotionally driven, like you said, but when you dip into some of the explanations, they aren’t primarily emotional driven. I definitely think there are some people who believe the earth is flat because of emotional factors or because they just want to be contrarian, so yours and @Matt’s points still stand on their own.
In fact, @Matt, what you described definitely exists! I’m a very logical person, and I tend to question everything. What you are describing actually fits in other areas such as the New Age movement, politics (both ‘right’ and ‘left’ circles), religion, social issues, etc. I don’t think what you’re saying is negative at all. It simply exists and people have to be aware of it in order to not be “sucked in”.
I’ve delved into a lot of what many would call “conspiracy”, and some is just as you say. Some of it is emotional manipulation, some is a lot of “what ifs” that have gone so far as to have nothing that could back them up. But other things are put forward with logical/fact information to back up what they are saying, while others could be plausible though it requires the acceptance of what we think is true, may not be so.
So, you know, who knows? We can assert that something is “true” because “everyone knows it”, but if everyone knows a lie, does that then make the lie true? We like to lean on “science”, and yet the more we do that, the more we make science into a religion. We shouldn’t “believe” in science, we should always question science because that is what a good scientist does. Even things like “gravity” or the “big bang” are just theories because while we have a lot of information that says this is likely so, something new could come along and completely shatter our previously held “truth”. Of course, we don’t want to simply believe everything that comes our way either.
We humans tend to identify with information we see as true and real, and this is why a person will fight tooth and nail to deny the validity of information to the contrary, or even be adverse to questions that could break down their own belief. You see this strongly in politics and religion. We need to have something we see as true in order to have a stable psyche and world view, but it works against us when those views are challenged. It can physically hurt to have one’s world view cracked. So we recoil, we mock, we create names for those people to “otherize” them, etc., and these things actually allow us to feel good about who we are and what we believe.
It’s easier to say, “I don’t understand why someone believes X” than to seriously look into the topic for information, because even if you still don’t believe it, you will have genuine understanding and that could possibly lead to questions about your current beliefs, which we humans definitely do not want.
Not all people who believe in “conspiracies” are idiots, emotionally led, or feel powerless and lost, but some do. I think some people find interest in these things because they are curious individuals and they are asking “why” (which we should always do). But I think some get stuck on one thing or another.
And there are always going to be charlatans.
I hope we get some people who are deeply interested in the ‘flat earth’ theories to post in here, and not feel like if they do they will be treated poorly if they share their ideas. It takes a lot of guts to have opinions and views different than the herd. But there would still be the need to be able to explain why, for example, eclipses would happen as they do if the world is flat. That’s a valid question. Where do you get your information? How would this or that work?
I think the quote/definition @AyBee posted is one that we should all keep in mind so that we don’t also get sucked into confirmation bias. To ask ourselves “how could I be wrong” is an excellent strategy. It’s good science to always try to prove your belief wrong, to test your own theories or preconceived notions.
Honestly, I think that this isn’t done enough by everyone (myself included). In the end we make our own choices, we decide on what we will believe or not; and, honestly, I don’t care what people believe–but I do care why they believe those things. Is someone just parroting information, or did they actually take time to consider and seek?
So, hey, if someone did their due diligence on the earth being flat and they’ve decided that they think it’s true? Awesome, because I have a lot of questions. 
. Now I really need to know the title. 
. When I was reading ‘The girl in the box’ i was affected in a similar way …both about abduction Ive just realised.
