This is just an opinion from a middle aged woman with no kids, but not one of those "get off my lawn, you brat!" type of person.

Haven't been to WDW, but as a California girl, I have spent plenty of time in
Disneyland. Sorry for the length of this opinion, but I was pretty shocked that some grownups can be so insensitive towards kids, at a place which focuses a lot of its purpose at making things magic for little ones!
First off: I'm 100% against spoiling the magic for anyone - be it a munchkin or an ol' crusty broad like myself. Besides, it doesn't hurt anyone to just keep one's trap shut!
1. Pointing out "reality" (whatever that is) in front of kids, which could possibly spoil the magic, is no better than telling a stranger's child that Santa doesn't exist.

It's not my place to do that, nor do I think it's ethical. The physical nature of Marie, or Belle or anyone else at Disney is a conversation that should be between the parents and the little ones. What if the little one truly BELIEVES that it is in fact Marie, and not a person in a fur suit? So...when a person says, "Wow...must be hot in that costume!", the child may actually hear "That's not really Marie and this whole vacation is a big ol' lie." This can potentially open up a honkin' can of worms that neither the child nor the parents might be ready to deal with - especially not while on a glorious (and expensive) vacation!

It could potentially cause worry, stress and a crappy time. I refuse to be the person responsible for ruining anyone's vacation if I can avoid it!
2. For those of us big kids who may suspect that real life might include a few costumes (because Mickey can't be EVERYWHERE and needs a few helpers), I think staying quiet is an important courtesy. When we enter places like WDW, we enter a world that is somewhat outside of our own (but thank goodness gravity still works!!!) . We go on vacation to escape reality, not to reinforce it. That's what work, and bills, and other obligations are for. Yes, I know about those costume things - doesn't mean I have to believe it when I'm there. I'll save logic for when I go back to my 9-5.
Let's correlate this, ok? I mean, do you go on a cruise and tell some random stranger, "Hey...you know how polluted this water is, and that you can drown in less than 30 seconds, and that if you fall overboard you could possibly be lost forever and be shark food?" That is ridiculous - you enjoy the view, the relative security and the joy of being on vacation.
It's fun to spend a few days disregarding the real world and "what is possible/impossible" and that silly logic stuff. We are taught (erroneously in my opinion) from an early age that pretending is frowned upon, and imagination is wasted time. They tell kids that having an "imaginary friend" or living in a "fantasy world" is silly and wrong. I say Bulls....er....FIDDLESTICKS!

Some of our greatest thinkers - Einstein, Hawking, Kaku, Edison, Copernicus, Nietsche, Kant, Newton, Sagan, etc., were self-acknowledged dreamers, and look at how these people (and more) have enhanced our civilization and knowledge. I personally think that expanding my imagination at Disney means that I'm part of a very illustrious club! LOL
I would never try to spoil the magic by injecting reality in public. Especially when kids are there, doesn't matter if those kids are 4 or 44!
I think it's a good thing that the OP was separated from their sister for a short while - maybe it gave her time to realize that the rules are a smidge different in Disney! <<laugh>> I hope she returned with a better perspective!
Long live Marie, and everyone else!