Visa for family

I understand that you're both keen to get this visa as soon as possible, but without knowing where she's coming from, I can't offer any actual advise. From the Australian side of things, I would imagine that they have a process for visitors' visas whereby they are satisfied that she will be a genuine visitor and is not trying to come in under the radar to work or stay illegally. That process probably has a set time frame. If you're anticipating a delay from her country of domicile, then she would have to prod the officials from her end. I would be cautious that any letters from you or your doctors might backfire, in as much as they might have reservations that she might try to extend or overstay her visa in order to care for you if your condition isn't stable by the end of her allotted visa stay time limit. Good luck with your treatment and your sister and her visa.