Can I update my brothers iPhone and iPad to ios5 from my computer or can he only update from his own computer that he currently uses to sync

You can update it on your computer, but then when he syncs it to his own it will say this iphone is already synced with another itunes account bla bla and will ask you if you want to sync it with this account. It will wipe anything you put on it using your own itunes account. BUT it wont wipe off the new iOS5

Also with the new update you dont need to have a pc based or mac based itunes account anyway so it makes no difference. once they have iOS 5 he can delete everything on them and put anything on them without even connecting it to a computer.