You'll do fine at the bday party. Remember, it's not about you or her. It's about your boy. Treat her like you would a casual acquaintance--be pleasant, but keep your distance. If she tries to engage you in anything beyond the scope of the party, remind her that this time is for your boy, not for her or you.
As for her family, it sounds like you are handling them well. There is no reason to be anything but friendly and kind to them. You just don't want to share any of your strategy. They could be manipulated, unintentionally even, to work against your best interests.
You mentioned moving your money to where she can't get at it. I hope that means that you've moved it out of her bank. As an employee who is sleeping with the boss, she might be able to access accounts that don't have her name on them.
Peace and quiet is the order of the day. You're taking care of business so there is no reason to have contact with her.
