Kendo, I'm sorry that you didn't get any sleep last night. What I'm going to say is coming from someone who has brought new cats into my home...through adoption and/or fostering.
A week is not nearly enough time to introduce a new cat to a household. Cats need more time than that to get used to a new place, new person. They don't know that your brother will come and get them.
Do you have a "safe room" for these cats? If not, set one up right now and put them in there! Don't let them have free run of your place just yet.
Hissing and growling is normal. The cats don't know each other and they are establishing who is "top cat" and who is "bottom cat". Also, between the two cats tha know each other...they are vying for "top cat" status in this new house.
Try to think like a cat here.....Kendo. Cat's do not like moves. They are also territorial. In a house or yard...they have designated sleeping spots, eating spots, etc..... When a new cat enters the scene, or there is a new territory...they all have to re-negotiate.
Your brother's cats have been taken from their "territory" and from their people...and are now in a strange house that belongs to another cat and there is a strange person they don't know. They have also just came from a situation where a new baby came into their house....also upsetting. They don't know if YOU are a nice person, yet. They don't know that your brother is coming back for them. Pretty scary, huh? So, of course, they are going to hiss and growl. They are terrified. Once they realize that you are a nice person who feeds them and talks nice to them, and that your place is not bad and they work out the hierarchy....all will calm down.
It took my dh and I weeks to get our cats used to our foster kitty. We kept the foster is a safe room, and let him sniff and explore the house only when we were there. There were hisses and growls and foster kitty bopped Korben on the nose...but they settled down and co-existed just fine until Foster Kitty was adopted.
When introducing a new cat(s) to a household....think like a cat. Give them their own room/area that 's theirs and only theirs. This is their "safe" room. Keep food, water and a litter box there. Don't put the food/water and litter box next to one another because cat's don't like to eat/drink near where they use their bathroom. This gives the cats time to calm down and slowly get to know you, your cat, and your place. I'm talking weeks here. It's a doable thing...but it takes patience and a little time.
Let the cats sniff each other through closed doors. They then get used to each other's smell. Let the kittys out to explore the rest of your place when you are there for brief times. During that time, keep your cat in your bedroom.
I"m sorry that this is happening during your finals.
I would definitely tell your brother what's happening. Perhaps he can find a house sooner, or he can find someone who is not having finals who will foster the cats until he can retrieve them. In the meantime, I hope my suggestions will buy you some peace and relative quiet time to study and sleep.