I'm going to ask my roommate if her brother could not sleep on our couch while he's sick
Can you explain more about what you mean when you say he is ill?
I remember my position as a guest in their house and I'm sure they would let me sleep on the couch.
I see, but you weren't sick then and you're just looking out for your health today. You have the right to take care of your health and property, and you can't let that brother potentially spread all his germs everywhere and make you sick.
I'm really scared to ask them about it.
There is no need to be scared to speak up about this. You have a right to be comfortable in your home so you should say something about your concerns.