Yes, the sisters are unkind and cruel.

Firstly, when they saw the melodious little bird with Princess September, they got very upset and distressed and tried to get away with him by offering a yellow and green coloured parrot to September. The idea of locking the bird in a cage was also given by the sisters. They not only suggested her to keep her windows shut and be firm with the bird but also went on to say that if the bird died inside the cage, it would be ungrateful of him and would serve him right, thus, getting her rid of him. Later on, when eventually the Princess was compelled to set the bird free, her sisters mocked at her and made fun of her giving him the freedom of going out. The mentioned instances and situations reflect the harsh and insensitive nature of the sisters who were envious of their younger sister.