My sister-in-law recently discovered "Unschooling" and is considering it as a viable option for her daughters. Does anyone have any feedback on the method? How does it work? Is it effective?

Strictly speaking, "unschooling" is a method or philosophy of learning popularized by John Holt. It is child-directed, therefore, no curriculum or structured system of learning is used. If they want to learn math or other subjects, it is because they expressed the desire to learn it or found a need for it. It is not parent led or curriculum based. I cannot give feedback if it is effective as most of my homeschooling friends are still following a curriculum.