Depending on how old this new kitten is, and the fact that you are only cat sitting your brother's cat for one week, I would probably keep them separated anyway, for the entire time, simply because
a) your brother's cat might already be a little scared, having a change of scenery without your brother around
b) smelling and hearing a strange kitten around and not knowing where it's mother might be
c) he might not be in the mood to have a kitten jumping all over him
d) the kitten may also be very scared being in a new place, possibly just taken from it's mother and siblings
e) the first thing that might happen is the cat might attack the kitten because he doesn't know him, and he could seriously hurt him or even kill him, possibly unintentionally
f) your new kitten needs to be checked out by your Vet before meeting any other cats, to make sure it is healthy and won't pass on anything.
Mostly, though, since the CAT is just there temporarily, why put him thru the stress of coping with a kitty the whole time, when it sounds like you can just keep them separated. Then you won't have to worry about those darned ear mites! (except clearing them up from your brother's cat, who should be very grateful and thankful 
