[Of course he had mentioned it, careless of Sherlock to forget so quickly. He could blame the odd form that this questioning is taking, but the fault is still his for letting the information slip away.]

Thank you.

[Seems simple enough now that he has all the pieces, he can see it play out in his head.]

It seems fairly evident that the elder sister is to blame. Jealousy for her younger sister's looks and aptitude, the adoration she had from others. The sister was older, nearing the age of courtship, yet homely and unlikely to make a good match. Worry over this likely made her more bitter to her sister, and a row simply grew out of hand. Such an intense passion as jealousy often creates a frenzied attack, and stabbing is a way to mar any physical perfections.