Well when you think about it, unlike Kelly Diana did not have a warm family unit right after birth. She was passed around from one group to another. I still say she reminds me of those children in the Lost Boys episode in that regard; emotionally and socially dysfunctional, determined to create the family she didn't have growing up and caring not one bit about the consequences (meaning how it will affect others) in getting it. Certainly it seems she didn't have many friends, just people watching over her. Kelly of course had Nick's firends, all of whom showed Kelly affection. So if this love was imprinted on Kelly, at least he has a chance I feel to be a good person.
So for Diana, it's going to come down to whether or not she can grow above her own desires. Can she accept she cannot force her parents to be together? Can she accept that her mother prefers Nick to Sean? Can she accept Nick as a stepfather, and Kelly as her half-brother? Can she rise above cold-blooded murder? I guess we'll see soon enough.