I cannot answer your question about how long she has; even the doctors really will not know until very near the end. Even then...well, when my friend died of esophageal cancer, his doctors kept telling him that he had only a few weeks to live, over and over again for several months. They'd also told him that he had 6 months to a year to live when he was diagnosed, and he lived over 2 years. He, like your SIL, just did not pay any attention to the seriousness of his condition, and I think that kept him alive longer.
However, EVERYONE needs to have preparations for dying. Whatever your age and whether you are sick or not, you need to establish who will make your decisions for you if you cannot make them for yourself, and you need to let family and friends know what you want as far as funerals, obituary information, bequests or inheritances (who gets what), funerary flowers or donations to favorite charity, medical requests such as when or if to withhold treatments which will only prolong dying or whether to treat up to the point past brain death. There are many choices to make. EVERYONE needs a will, a general power of attorney, a living will, and a durable power of attorney for healthcare. I cannot stress the importance of making these arrangements.