How is differentiation to be made between sons and daughters in treatment and care, especially concerning financial support and the treatment of children?

The Fatwa's Summary: You did not clarify why your parents prefer their eldest son. The majority of jurists hold that equality in giving gifts to children is desirable, and those who consider it obligatory permit preference for a legitimate reason, such as need. If your parents' preference for their son is due to his need, then there is no sin upon them. They might not even see themselves as unjust if there is no (apparent) reason. If the preference is indeed an injustice, they should be advised kindly, bearing in mind that admonishing parents is not like admonishing others. Filial piety is obligatory towards parents, even if they err. It is recommended to maintain affection within the family and to close the door to Satan, who seeks to sow discord.