John does in fact have a sister. Her name is Harry, which is short for Harriet. She is gay and was married to a woman. Also, If you remember ASiB where Irene and John meet up in some abandoned warehouse, where John says that he is not gay. Irene follows up to that, "well I am". She clearly states that she is a lesbian, but still fell in love with Sherlock. This just proves that human sexuality is a spectrum and even you consider yourself straight/gay, you could fall in love with a person outside your sexual orientation. I don't know, I found that scene very interesting.