What your mother-in law said is (almost) true. If she has 2 children at least 66,33% of her estate will be divided equally between her 2 children, by law. It's called part réservataire. If she has 3 children or more this part réservataire will amount to 75% of her estate. The remaining 33,66% or 25% of the estate can be willed to any person or organization she wants. If no will is written in the proper form though, the entire estate is to be divided equally among all children.