What if they were children of different parents but both of each set of parents were killed, and John Winchester took them in and raised them as his own? Keeps the raised-as-brothers angle but technically lowers the threshold of the incest angle. (I say technically because they wouldn't be related by blood, but there's been a study that says the negative incest reaction is more a factor of being raised together than being blood relations. ...I kinda wish I'd webloc'd that, now.)