I don't know if this is the same "friend" from your other post that I responded to. I will assume so.
If the SD's behavior is acceptable to the dad, then there really isn't much one can do to change it. The parents can make house rules that they will both enforce, though. (Like no sneaking out or sleep overs.)
As for the SS's behavior, that is a different matter as it DIRECTLY affects and harms the stepmom. If it were me, I would clearly and in no uncertain terms tell SS that behavior was unacceptable. And if I told my DH and he took no action whatsoever, I would consider separating until the kids were out of the house.
Sounds like a lot of immaturity in one household. Sometimes the only way to stop the cycle is to get off the merrygoround, ya know?