Is the fate of the sisters just?

Yes, the fate of the sisters is just. Everything they did to Psyche was out of jealousy because she had a better life then they did, according to them. In the beginning of the story they found out that Psyche lived in a magnificent house and had a nice husband. They saw all the jewelry she had and they wanted it. They also told Psyche that her husband was an evil monster and wanted to eat her. So they told her to kill her husband while he was sleeping. They did this because they didn't want her to have a better life than them. " Tunc nefariae mulieres puellae cogitationes invadunt: 'viam quae sola ducit ad salutem diu goitatam monstrabimus tibi"(li 23-24). In the end Psyche lied to them and told each one of her sisters that the man she was going to kill was Cupid himself and he left her and wanted to marry one of the sisters. When the sisters heard this they went to Cupids house in a heartbeat without thinking about how Psyche felt. Since Psyche tricked them they jumped off the cliff and were killed.