It`s worth trying Coco as sometimes they are bickering over cage accessories, but as the others have rightly pointed out, if one is being bitten and attacked, don`t hesitate to split them. Put human feelings aside because hamsters don`t think the way we do remember and I totally understand why you want to keep them as a pair, but even sisters can fall out. Just because they are siblings does`nt always guarantee they will always get on. It`s a shame, but that`s how they live their lives. It`s nature. If the one doing the attacking even starts to chase the other one again, get her out right away and set up a cage for her as she was used to in her old cage, or better still, remove the bully and put HER in a new cage. Why should the bully keep the old habitat?
Hope things settle again, but blood being drawn is usually a bad sign. Fingers crossed for you and your girls.