You pull out the butter knife, realizing how small it is compared to the bulk of the wolf. Still, it might be able to pierce the skin and you thrust it out towards her chest. However, slips through the fur and doesn't even get close enough to scratch her skin. She blinks at you with confusion, before taking the offensive. With a snarl, she rams her body against yours and sends you skidding across the cobblestones. Her lip curls and her hair raises as she lines up for another attack. You try to calculate your next maneuver, but she pounces before you can gather your wits.
Her claws slice at your clothes as Ell pulls you out of her cross hairs. She snaps her jaw, her whole body shaking with her growl.
"She's lost to the werewolf venom now," Ell says, dragging you down the street and pulling you around the corner. "You can't turn her back now, she's too far gone to the beast mentality."
You try to protest, you beg and plead, but Ell is far too strong for you to fight. After they plant you upon a plush, velvet seat in their carriage, they wave a hand before your eyes and then everything fades to black.
You never find the need to forgive Ell for that night. You want to be angry at them, you want to blame them for leaving your sister behind, but they kept true to their word, just as they've always been true to you. They offered you a shot to save your sister, but when you failed, they did what they could to protect you instead. You know the only person to be angry with is yourself. And ever since you received word that Gianette died that night at the hands of the duke, you haven't been able to find it in you to forgive yourself. In fact, you accept Ell's invitation to become a vampire just so you can wallow in your guilt for eternity as that's the only punishment you deem fit for your crime against your sister. To die is to escape the pain you feel everyday and you will not allow yourself such a luxury.
***
You've found one of the unsuccessful endings for this branch of the story... Why not go back and try to snap your sister out of it some other way?