Gigi's right. Your mom's playing you and your siblings to get everybody on her side. She wouldn't have told you such sordid details that are, frankly, none of your business unless she was trying to fill your mind with hatred.

No, your dad's not a saint, but neither is your mom. Your best bet is to be Switzerland here -- stay neutral, and make it clear to your mom that you don't wanrt to be dragged into the middle of this.