If you're going to do this more than once a year, it would probably be cheaper to own, but if once a year or less I'd consider renting. There are places to rent cheaper that BMW, some private companies that offer a variety of bikes, even BMW's if you have to ride one of those
. If it's going to be one brother at a time, maybe you all go in together and have him buy a second bike and keep it ready for whoever comes over next. If you're all coming over at one time, obviously that's out.
I'm living an hour north of Ramstein at another base, and had a friend fly over and rent a bike in Munich for a week of touring in the Alps last year. Fantastic riding, don't skip out on really doing it.