If I sent a Snopes link, it's because I don't want my friend to look stupid for propagating a hoax. It's not because "I know more/am better." You wouldn't want a friend to walk around with her dressed tucked into her pantyhose, and some of these emails/facebook posts/images are just that--potential embarrassment. Of course, I email the link in private. The same way I would quietly pull her aside and inform her of a pantyhose mishap. I wouldn't scream, "Holy heck, your panties are showing!!" =)
I think you make a good point, and a good analogy.
Another reason why I sometimes send people a Snopes link, is to avoid them from being upset, to reassure them. For example, if I see people being really upset about, to give an extreme example, Bonsai kittens, I am certainly going to tell them there exists no such thing.
We have an extra complication here, though. The OP is not sure whether the niece is serious or not. So if you allow me to run along with your wardrobe mishap analogy, I would say that this seems like a case of "Hmm, does she /mean/ for her cleavage to be so deep, or is she unaware that her shirt has unbuttoned?"
We had a discussion about a wardrobe malfunction a while ago on e-hell, and some people suggested luring the person towards a mirror, to make them find out for themselves.
There might be ways to accomplish something similar in the topic at hand, as well.
OP, you could reply something like: "Oh I saw that picture too, I found it on ..." and then include a link to either Snopes, or if you think the mere mention of "Snopes" is too much "haha I know better than you", you could link to certain newspaper articles describing and criticizing this whole conspiracy theory.
If it is what I am thinking of, there are quite a few sites beside Snopes who have written about it.
Alternatively, you could post something about it on /your/ wall so that it will appear in her feed. For example, a link to an article about a man who has been harassed about his alleged "acting" and "lying", and how he is suffering from this.
Other posters who are wondering what this is all about: I understand, if it is what I think it is, why the OP wouldn't want to go into detail, because it is rather politically loaded. But if you take a few crucial words from the OP and google "conspiracy hoax recent tragedy", you will quite certainly be able to get an idea what this is.
Of course, for all I know I am completely mistaken and talking about something completely different... But in that case the main gist of my advice to the OP still stands, and disregard the rest.