I am attaching the scripts.
- chbrn is a script that gets and sets the brightness, put it somewhere in your $PATH.
- sony-brightness-up and sony-brightness-down go to /etc/acpi/events. Run acpi_listen in terminal, and get the events when you press Fn-F5 and Fn-F6. Edit those files to match the events. Since your laptop is also Sony, I believe they will be the same.
- sony-brn-down and sony-brn-up go to /etc/acpi. Change the brightness levels as you like. I set 8 levels, as there are 8 brightness levels of the laptop.