Props to you for taking in the critter but he should have never been put into your rack, anything new goes into QT well away from your established, healthy critters. I'm guessing your brother's snake got the RI from being too cold for too long and he was sick when you brought him home, he just didn't show symptoms for a while. That snake needs to stay in QT for at least 90 days after he stops showing symptoms. Hopefully the antibiotics work.
Does your mother understand just how large retics can get?Recently he's gotten a sub adult Boa Constrictor Imperator, and a Reticulated Python, both my mother's doing. At this point, he keeps snakes for a month or so and is bored of it and either sells it to me, or doesn't mess with them except necessities. It's basically a case of him not needing any animals to take care of, but wanting and then my mother happily getting them for him. I've made it clear since he got the Retic that there would be no more animals brought in unless they were mine or my fiance's, since we can actually take care of ours.