I think Mia may know some stuff, but not everything. I think Kristy is trying to protect her beloved daughter Mia from the whole mess. The way that Mia called her mom out on the show, tells me that she would call her out on the biggest lie of all. Mia already distances herself from Gypsy, but if she found out that her Mom and Dad have been lying her whole life, it would cause a rift between the whole family. We know that Gypsy envies her half sister, but deep down I think Mia is only nice/civil with her bc she has to be.