L.L.
How terrible! I used to work at the Ky Humane Soc & saw people turn in cats (in tears) b/c the animal was destroying the home, causing family fights, etc.
If you spoke to a vet, they might ask to see the cat in case of a UTI, which causes the cat to spray everything. Or if the move-in of the brother is recent, the cat may be stressed out from having to go to a new place. Does the cat understand where the litter box is? (yes, I'm being serious ... sometimes when you move it - even to a new room - cats get confused and testy.) Is the cat a boy? Perhaps it should be neutered? This helps them to stop marking territory.
Why can't the brother keep the cat confined to his room? shut the door? esp during the day when you aren't at home? I would worry A Lot about letting the cat get near the baby's room/crib. My cats were good about leaving my son alone while he was In the crib or bassinet, but like to jump in while he was out. This got everything hairy, so I had to make it clear that is wasn't acceptable.
The smell is awful ... I've tried baking soda, etc. but it doesn't always help ... perhaps stop in a Feeders Supply or your vet to see what they recommend. There are lots of products, but I'm sure most aren't that wonderful. Animals tend to go back to the same spot over and over even when you wash it ... so sometimes you have to throw the item out so they can't smell the area.
Good luck!!