Is it permissible to pay zakat to cover a sister's university installments if the father is financially capable but does not spend on the family?

It is permissible to give a student of Islamic knowledge from Zakat according to their needs if they are unable to combine seeking knowledge with earning a livelihood. Jurists have explicitly stated this. Some scholars have also gone to the extent of permitting giving Zakat to a student of worldly knowledge if they are seeking beneficial knowledge for the benefit of their religion or their worldly life. Therefore, if your sister is seeking Islamic or beneficial worldly knowledge and is unable to combine studying with earning a livelihood, it is permissible to give her Zakat to cover her tuition fees.