Originally Posted by TheSquealer

The only power she has over you and the only influence she has in your life is that which you and your girl allow her. This is not about "the mother in law" - you need to sit down and talk with your girl about the boundaries you guys need to agree on, set and stick to.

This is going to sound amazingly horrible but people are no different than dogs. Show weakness, you get tested non stop. Establish dominance and be consistent and you're in control. Be the boss, establish boundaries and enforce them.