Advice about cats needed here.
Jamestown, I haven't read all the replies but I can relate my experience to you. I took in a little brother and sister duo. They were a bit younger than 8 months at the time. The little brother was and still is a little terror most of the time (he's also loveable as all get out!). He just plays too rough. He was so rough with his sister, that she was very nervous all the time. We found her a great home where she was an only "child" and treated like the queen she always knew she was. :D
Bottom line, I think I'd go with the older female instead of the brother. She may be able to handle the "little" one better. In our home (4 cats, 2 males, 2 females) the girls won't take any sass from the boys. The girls rule the roost! The boys try, but one hiss and they run off. Since the brothers are known to fight all the time, I don't think that will change.
Of course, they're cats so everything I just said might, quite possibly, be hogwash! LOL