I too would build a bat box and place it near where they are gaining egress to your brothers house, then observe when they are coming and going and where it is exactly. Board up or seal it up and hopefully they will use the provided bat box as a new home, if not move on to greener pastures. Having a pet bird now I know what a PITA cleaning guano can be, but on the bright side most larger pet stores that stock bird supplies have cleaning compounds that aid in removal.

-Bill Liptak