You cannot bang a guys' ex-girlfriend, ex-wife, sister, or half sister, without his explicit - crystal clear - permission.
You can, however, mack on the ex-girlfriend or get some titty-action from the ex-wife, as it comes under the wide, wide umbrella of the 'It Was the Alcohol' Rule.
"Get out of the truck."
This exchange:
Tommy: (on the flowers being sent to his daughters' hospital room) Another one!? Nurse: Yep. This is the fourth. They're getting bigger. Tommy: Any candy this time? Nurse: Sent it to the cancer kids.