Actually, it isn't true that we all dress the same. Personally, I make a lot of my clothing and jewelry by hand, so it is one of a kind. And I have worn bright tie dye, before. I don't stick to a strict, all-black outfit. Some goths do elaborate make-up and hair, while others of us, like me, do unique clothing, and normal makeup. Some goths wear skimpy cloths, others are very conservative. Some have piercings, some don't. Some like tattoos and stuff, others prefer historical dress.
I kind of got into goth style before I even knew what goth was. Many goths start inventing their own style, then discover a subculture of similar people. This does not mean we are identical. Many younger goths are what is called "baby bats", and they tend to fall into clichés. A band shirt with bondage pants does not a goth make. As they get older, though, many of these young goths branch out and get creative. Instead of dressing like their peers, they begin to make their own unique style, and listening to their own music, not just the big goth bands. I know a few artists with less than a thousand views on their songs. Most goths that are not baby bats or snobby elitists feel free to experiment with lots of colors, jewelry, music and more. The art and style is important in the subculture, and therefor experimentation is encouraged. I have never copied the outfit of anyone else, and have never looked like any other goth that I have seen. Frankly, I can't afford to buy the mainstream goth clothes. Many of my designs are inspired by nothing but personal taste, not mainstream goth. For a while I worried that it meant I wasn't really goth, just a poser, but then I realized that adding your own creativity is what the culture is about. I do lots of typically "ungoth" activities, like sports, and I just in general do what makes me happy. Really, you shouldn't try to hard to be like other people. I don't like Siouxsie very much, for example, but that doesn't make me less goth. I can feel confident working a Star Trek shirt or a cute modern dress into an outfit, without changing who I am. Some days I just want to wear boys cargo pants, a loose shirt and a neon hat, and skip goth altogether for a day. Your niece sounds like a clueless (but we love them anyway) baby bat, and will very likely either quit the subculture or evolve into a full goth.
One big thing to keep in mind: not all goths are young. Some of us are quit young, yes, but the subculture started in the early eighties, and there are people, old now, that have been in it since then. Many have raised families of their own (some are even old enough to be grandparents), and have had successful lives. There are gothic nurses, teachers, engineers, and pretty much anything else you can imagine. Personally, I hope to be a theoretical physicist someday, and do grant writing on the side. Goth isn't always an adolescent trend, it is quite often a lifestyle choice embraced by many different people.I am sorry you see us in our clichés, and that you have not met many of the unique, wonderful people involved in this diverse culture. It is a place where we can be ourselves and not face judgement from those that are different, because we know we are all different in some way. That is our ideal. If a goth wants to wear hot pink, or dance to Bollywood music (I love Indian music and jewelry), or grow old and raise children, or listen to pop, or watch mainstream TV, that is okay, or should be to those goths that aren't those obnoxious snobby "gothier than thou" types.
Please be patient with your niece. She is just trying to figure out who see is. Most young goths fall into stereotypes because that is all they know, if they are not second generation goths and do not have any gothic role models. She will either grow out of this (a distinct possibility for many), or grow into a unique, creative, beautiful goth girl, who isn't afraid to experiment with things that aren't mainstream in the goth world. I never had "goth buddies", so as the only goth in my school I guess it was easier for me to get creative and do my own thing, without worrying about elitists, but someday your niece, too, will find her own path, totally unique. If that path is uniquely goth, then good for her. Unique and goth can go together.