The Orders follow Elective Succession and so any of their male courtiers can become new Grandmaster. It's obviously bad thing that it happens, but I think game can't really make a difference between Elective Law in Knight Order and Elective Law in Pagan Chiefdom. If it could, pagans (and moslems) could be excluded from inheritance in Knight Orders, Bishoprics and likely Republics too. Actually, I guess it would be best if all different religion people would be excluded from Elective Succession, but would that be possible to code?
Or worth the effort?