Well, as I understand it, she DID know she wasn't supposed to eat th' apple, God had told both of them stay away from it. They apparently felt shame at being naked so to THEM it wasn't okay. I'm not sure it's as much sin as it is the introduction to ego.
Nope...the Genesis account tells us that God told Adam but makes no mention of Him telling Eve about the tree, so evidently Adam conveyed the command to Eve...Gen. 2:15-17
It wasn't until after they both ate from the fruit that they felt ashamed of their nakedness...Gen. 3:7