Let's call it "superior technique and experience." Sorin has been fighting with a sword for six-thousand years. Saber has existed for only about fifteen-hundred years. Factor in that Sorin too has superhuman fighting ability, and we can wave away a parry.

What I do find ridiculous, though, is the expectation that I'm apparently supposed to tailor my roleplaying around the fact that your characters have the irresistible urge to wantonly murder party members as they see fit. As much as you keep saying that you didn't want to do this, you still kind of... did. You can say that to not kill him at this point would be out of character, but I doubt "delves into a mindless killing frenzy at the implication of something evil afoot" is a trait of either character.

Anyway, this can still be worked around. Sorin's explaination (deliberately OOC, accidentally IC) should have struck a nerve with Saber. Perhaps reminding her of her own hardships will stay her blade long enough for this to develop into a tense (yet bloodless) argument. 

As far as Archer goes, I'm at a loss. The player historically refuses to listen to anything I say, so the character likely will not, either. Any attempt to reason with either one is probably a wasted effort.