Tell your sister to stop with the 'pretending' that she 'digs' him. It will be bad enough when he realises he's been played. Do not continue. You found out what you needed to know. How much more do you need to know to make you angry enough to proceed on the divorce path. Any more knowledge (about his unfaithfulness) will only hurt you. Keep yourself informed RE financials and safety for you and your kids. Nothing more.
Get a safety plan together. Have a bag packed in the car. Have some money tucked away. Have a safe place to stay that he either can't get to or doesn't know about. Get a prepaid phone with a different number and do not let him know about it.
Bridge has got a great list somewhere. I think she may have already posted it here.
** Purple
As soon as you trust yourself you will know how to live. Goethe
