So far as I know the brand does not prevent conceive a child but if they suspect a member of their clan (branded) one was kidnapped for breeding purposes and rescue is not done immediately (or be possible) they may kill outright the kidnapped brand member to keep their bloodline exclusively with them (as using the brand for that purpose 'seals' the bloodline not be passed on at all, even from a corpse). A unbranded heir in such missions are unheard as they are not send out to be genin (Hinata was expection for lot of ways because she was former heiress who was assigned to be genin to serve her clan).