If it wasn't for that we would either be homeless or be all living in my grandparents' house

Or you could become tenants from a wholesome small landlord! I wonder why that wasn't in your possibilities?

Are we the assholes if we rent our house in order not to force my parents to work 200 hours a week since salaries here barely reach 800€ a month?

So the people renting your apartment will be the ones working 200 hours a week instead? The lifestyle of your parents depends on other people paying them rent and you still can't understand why private rent is theft?

what we need is strong national regulation, not banning renting houses altogether

The "stronger national regulation" needed is the expropriation of rented housing to a collectively owned rent organization, and the masse-construction of affordable housing for social rent, and the rent of all of this housing stock at production+maintenance costs.

To be clear: most people would do what you're doing in your situation. But the fact that your family is escaping overwork and poverty through renting one of their flats simply means that another poor person who can't afford to buy a flat is subsidizing their lifestyle. It's not that your family are intrinsically evil people, it's that private rent is exploitative by its very nature.