Richie mentioned
> … skin color is immediately established as the murder motive.

It could be skin colour, but hair colour seems more likely. Fair skin was certainly valued at one time, but the contrast is generally reckoned to have been between rich people who spent most of their time indoors and poor people who spent a lot of time working outdoors and so got suntanned. Two sisters would probably have had very similar lifestyles, so their skin colours would have been much the same.