Ipso facto, using up your oxygen, you know I'm shallow

Happy National Day to all fellow Singaporeans reading this... spent the earlier part of today listening to the directors' commentary for About a Boy - man, the Weitz brothers are really cinematically literate! I do like their oeuvre: American Pie is one of the only so-called gross-out comedies that holds up with repeated viewings, probably because it actually features powerful women, and I found Down to Earth a good remake of Heaven Can Wait. But anyway, the commentary is interesting, because it's a good reminder that directors constantly think of numerous things in crafting their movie even if the ordinary moviegoer can't be expected to get them: the Weitzes talked about imitating Scorsese's shots from Casino, and the switching of lenses, and all sorts of interesting technicalities. Question: why is it I can think of lots of pairs of brothers as directors (the Weitzes, the Farellys, the Coens), but not any pairs of sisters? Is it just because there aren't enough women directors in Hollywood? Pity, really.