Flip the Switch
Until they flip a switch on you, you believe you know someone. We all have that switch, but it is more noticeable in some of us than others. You may get to know someone for years and come to think you have a solid grasp on who they are until something happens to them or they ultimately show their true nature.
They feel they know and like their mother-in-law after one day. Then, a few years later, she transforms into an irksome individual to be around who one-ups everything and pretends to be someone in front of others. Something must have occurred in her life to cause her character, or perhaps she was always this way.