Intercape budget liner seat size

I would like to submit a formal complaint. I booked a ticket for my sister who is a mother to me from grahamstown in the eastern cape to Potchefstroom in north west. I booked a mainliner only to be informed 2 hours before departure that it it will be a budget liner. Which is not a problem the problem is the size of the seats, half of my sister's body was not fitting in the seat even the aunty who was bigger then her was having a challenge. They have to drive more than 12 hours sitting uncomfortable. How do you treat ppl like that.