Both the "good route" and "bad route" have a special ending scene. The sisters begin the game on bad terms with one another. In the good route, you need to fix their relationship, and in the bad route, you need to make it even worse:
Good route:
Diana is angry with Elizabeth because she believes that Elizabeth stole her ring. To fix this (good route), you should return Diana's ring to her and let her know that you found in by the pool. Diana will realize that she had made a mistake by accusing her sister, and seek reconciliation.
Elizabeth, on the other hand, is angry with Diana because she believes that Diana told her father that she was doing drugs. In order to fix this, we will have to convince her that it was not Diana, but someone else who snitched on her. To do this, we will need Bob's help. After you've won Bob's friendship with the rum, show him the typed note regarding Elizabeth's drug use. He will be able to forge a letter with someone else's handwriting, but needs a sample. For this purpose, give him the handwritten letter from the brother. Once he creates a forgery, present the new letter to Elizabeth. This will convince Elizabeth that it was her brother, rather than Diana, who informed her father. Elizabeth will then apologize to her sister.
Now that you've patched things up between the sisters, finish the game through the good route and enjoy your extra ending scene.
For the bad route:
Similar to the good route, but with a couple of changes. This time we need the sisters to hate each other. When you return Diana's ring, tell her that you found it in her sister's room. This will solidify her hatred towards Elizabeth.
For Elizabeth, we're going to remove any doubt from her mind that Diana set her up. When you ask Bob to make the forgery this time, give him Diana's notebook, then give the new note to Elizabeth.
Once you finish the bad route, the sisters will turn on each other, giving you an opportunity to take advantage of them again.
