^^For me, it's the fact that Sayo (right) used to hate Hina (left) because of her (Sayo's) inferiority complex compounded by the fact that Hina loved her sister a lot that she copied everything that she did and was just better. However, after several events, Sayo's gone through immense character development and now the sisters have a good relationship, enough for them to go to each other for advice.
Plus, they're twin sisters which is a big plus.