When you do get your BFP Brixton I can imagine how hard it is going to be to tell her. I really feel for you all. Is your sister willing to have some regular counselling sessions?
Well, as ever the pessimist am not really a lucky person I found this on Dr Google as another reason AF has not arrived:
"One of the main causes of a delayed period (extra-long luteal phase with high temps) with negative pregnancy tests is a luteal cyst. In a nutshell: when you ovulate, your egg ruptures from a small follicle. This follicle is known as the "corpus luteum" (literally means "yellow body," named for its color, and is the origin of the term "luteal phase.")
"Anyway, the corpus luteum produces progesterone. In the event of pregnancy, this progesterone will support the growing embryo until the placenta takes over and makes enough progesterone for itself. Then the corpus luteum shrivels up and stops functioning.
"In the event that there is NO pregnancy, the corpus luteum also shrivels up and stops functioning-- ending the luteal phase. The drop in progesterone causes your period to begin.
"SOMETIMES, however, there is no pregnancy, but the corpus luteum turns into a small cyst and continues to secrete progesterone long after it's supposed to stop. This is called a luteal cyst, and is responsible for too many false hopes!"
Hmmmm...