I'm a bit skeptic of that story, to be honest, given that there is absolutely no proof that Emily and Branwell had an incestuous relationship. Many writers and biographers of the Brontes have insisted on that idea (making various claims from the one where they were lovers to the one where Branwell raped his own sisters) , but I think they did so to enhance their own popularity (because everyone loves a good scandal). There was no actual evidence that such a relationship existed, though.
As for her suffering from any sort of mental disease, I think that's just another example of our modern day society trying to impose ideas on the past and to explain it. Given that there is precious little information available about Emily's life, I find it a bit hard to believe that it was enough for doctors to actually find she suffered from any sort of mental issue.
Thanks for reading and commenting :)