This thread reminds me of "blood is thicker than water," and how we've come to misuse the phrase over the years. Currently we use it as if being blood related matters above all (it doesn't necessarily mean a thing though, if we're honest, blood relatives are often the worst thing for us), but the original version was "The blood of the covenant is thicker than the water of the womb," anyway, which is to say shedding blood alongside each other in battle is a stronger bond than that of family ties. It's a phrase for people exactly like these two.
His amnesia is worrying though, because it's almost definitely there to generate drama later on. Possibly to put them on opposite sides of a conflict. That'd be a pretty typical play for Chinese martial arts story to go with.