Unfortunately, predicting how long someone can live is not a scientific affair. Even specialty doctors can only guesstimate. There are many factors that are hard to measure, such as the strength of someone's will to live, faith or other such intangibles. While the body may wear out, sometimes the mind keeps going for a time afterwards. Your mother-in-law certainly sounds ill, but whether she will last another month or another year is not something anyone can really tell you for sure. I am dealing with similar questions about my own mother. She gasps for breath with any type of exertion (even walking down the hall), but I have no idea how long she'll last like that.
Hang in there,