I understand why they love the white haired sister soo much, in front of asharte she might have been the villian and mean, but to them she was always a sweet girl, they were her brothers for 18 years and I don't think it was ever truly revealed how evil she was because she was labeled as a victim even by asharte. It's just not as easy as we think it is to abandon someone who they thought was their family for so long, especially if they still think of her as the young girl who they adored.