Is it unholy to enter a relationship with your ex-girlfriends sister?

You are a selfish SOB, or perhaps a lecherous pig. Have you no heart? If you had parted on good terms, or she left you, and SHE was OK with you dating her sister, then it would be alright. But this is not. And her sister should not be with you. You are both doing wrong. You are to be kind, and a person of compion. Where is the kindness and compion when you take up with her sister when she is crying her heart out over you.. They should both dump you. You are a jerk.