Uhm, off topic but here are some pointers for you: Itch.io is just a marketplace for games. You can make a game using any kind of game development software and offer it for download here.
If you're talking about making visual novels, which are actually a good place to start for a dip into game development, RenPy is a very popular engine for creating visual novels and it's easy to get into.
Though you have to keep your expectations in check, you can't create a game like this by just using RenPy. But let's not take too many steps at a time. Best of luck to your cousin!