As far as the Turkish law is concerned your husband is obligated to support his children "attending university". The normal case is " up until majority" which is 18 years of age. Yet the children attending university ("continuing education") is an exception as many court cases suggest.

You note that your husband ("and father-in law?" I could not follow the line here) is not working. Then courts will consider his assets. If no assets then it is quite unlikely that the children could obtain something.

You point out that you own a small business so it is probable that courts will consider it. It does not necessarily mean that your husband should pay all the university fees. The law states that the nature of support is "contributory" depending on the "circumstances" including their mother's condition.

Sincerely