We often hear that parents must treat their children equally. If it is so, then why had Prophet Yaqub's (as) sons felt that he preferred Yusuf (as) over them?
Prophet Yaqoub (AS) was treating all his children equally but some of them felt jealous of Yousuf who appointed by Allah (SWT) as a Prophet. The fault was from those sons and never from Yaqoub who did not discriminate at all but was always just and kind to all of them.
The bad feelings and jealousy of these sons lead them to commit the crime which they committed against their own brother Yousuf.
Wassalam.