I mean, not sure wtf is wrong with his younger brother, but I would at least get some puppy pads and put them on the bathroom floor, that shit is disgusting. There's absolutely no way I would deal with cleaning that up on a daily basis.
I don't know the full story here, so I don't want to shame the OP parents here or anything (brother could be developmentally disabled or something) but, I can not imagine a scenario this would have been allowed in my house growing up. I mean, my mother would have straight up murdered me if I did this more than once on her clean bathroom floor. Right to jail, right away.
These are the pads: https://www.amazon.com/gp/product/B004AP9Y58/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
WE HAVE THE TECHNOLOGY - Only Simple Solution pads come with InstaShield Technology, which allows these dog pads to contain up to 6 cups of liquid while neutralizing unpleasant odors. Protect your floors and your sense of smell.