I understand Sihwa's pain completely; it's awful to have someone you love say such things, but he must realize that Sohan must handle this himself because it's his father, and he must be the first to take action and establish boundaries with his dad. Sohan's remarks were harsh, yes, but he only said those words because he believed it would be best for Sihwa, even though it might not have been the best course of action given that Sohan might depart soon. (On the other hand, I can't blame Sihwa since he was basically raised in a box and not knowing how to deal with his emotion and outburst.
The apple in the Bible represents both good and bad knowledge, and if Adam is the bad apple, he opens up a world of both good and bad emotions, I've noticed in Eve. We know that eating the apple was a sign of disobedience, which ultimately resulted in death.