I think my biggest beef with the Narcissa and Bellatrix hair thing isn't so much that the genetics for those two must be weird - because that does actually happen, sometimes, as your anecdotal evidence makes clear. It's that JKR doesn't seem to have put that much thought into it (which, you know, really only drives me nuts because I'm the kind of author who draws diagrams and stuff. So it's all on me, and not her, but still!). I mean, why do so many of the married couples have matching hair, even when it means that siblings don't? And why do so many people have black hair or black eyes, when hair and eyes are generally much more likely to be very dark shades of brown than truly black?

And yes, obviously Snape is better looking and more attractive... ;-)