Need Opinions!!!

Okay, so me and my identical twin sister have been working on growing our hair long and healthy. So for about a year we have been quite strict on keeping our hair healthy by avoiding chemicals, avoiding heat (straightners and curling irons), and using good conditioners.
However, we go back to high school in two weeks, and me and my sister always like to dress alike on the first week of school.
BUT,
about two years ago, my twin decided to dye her hair blonde, but I didn't... and this was before we started taking really good care of our hair.
And then the year after that, we both dyed our hair darker so that we would match again, and our hair looked surprisingly the exact same, but only for a short period of time. By the end of the year, the blonde in her hair started to show through again, and my hair also lightened a little to its natural brown.

But now to the point. At first we were gonna dye our hair darker *again* for school which is in two weeks, but then she decided not to because she read somewhere on the internet that dying hair darker after dying it blonde will risk it falling out. I don't really agree with that but my twin does and now she doesn't want to dye her hair darker anymore and match with me.

So I wanted to know if anyone knows if dying your hair darker after dying it blonde will really give it a risk of falling out. And I know dying your hair is bad for it, but it's not as bad as making it lighter.
A don't really think that if my sister dyes her hair darker again that it will damage her hair that bad. Besides, she only died her hair twice in her whole life (I dyed mine once).  

Here are some pictures of our hair side by side. (I'm not sure if the images will show up so let me know if they do or not and I'll try again. Or try copying the URL and pasting it into a new tab.)
(my twin is on the right. I'm on the left)

Dying your hair with ammoniated (with ammonia) hair color will damage your hair, hands down. It blasts open the cuticle with ammonia and the pH of your hair is never restored back to normal.

If you want to go darker, go to a salon that carries Organic Color Systems. They are ammonia-free and your hair actually gets healthier over time for using it!