User blog comment:Battlefield4us/3 Days Left - Blake and Yang/@comment-90.218.215.7-20140721185528/@comment-50.197.190.126-20140721201927

Blood related doesn't necessarily mean they're aren't half-siblings (sorry for the double-negative). As long as they share one parent, that technically makes them blood related. Unless there's a quote I'm missing of Monty specifically saying "They both share the same parents," I'd say half-siblings is a reasonable theory.