If you think "natural" in a hard, unyielding way, even showering with soap maybe be seen as unnatural. Making yourself smell good with perfumes and lotions is unnatural, grooming and plucking body hair is unnatural, flossing with mint floss etc... So I think it's useless to try to define in stone what is being natural.

That said, your sister is doing body modifications. It's not really much different from piercings or tattoos, or dying your hair with henna etc, and she's trying to feel good about herself. As much as we want to, we ALL are under pressure from society and ourselves to look in a certain way, and that "certain way" is most definitely well defined by (sometimes ages old) strict standards.

I'm sorry you've been feeling down, no one should feel that way. You are under no obligation whatsoever to stay unhappy with your appearance. If "natural" doesn't make you happy and confident anymore, you can just start doing whatever body modifications you see fit and safe for you.

However, if perhaps you want to study the source of your unhappiness with your looks, going to therapy and maybe reading a few books about beauty standards and self image could help. Your sister is not doing anything wrong, and neither are you, no matter if you decide to follow her methods or not. Doing body modifications doesn't make anyone a phony or fake - god help me, no one ever called the fake for having tattoos. We are subjected to time, sun, scars, sagging etc and some people feel good about themselves with it all, while others don't. I do believe personally that comparing yourself to anyone else, sibling or not, is a very fast way to make yourself unhappy and angry.

Wish you the best!