I've found this too! In my extended family, and in my friends families.:
I've found the families who have the most kids get along better as a unit, regardless of the combination of the sexes. I know of one 'one boy one girl' family where the siblings have always gotten along, and thats largely because the boy was older and gay and they were close in age. The number of diva smackdown style matches I've seen in two girl families is phenomenal - especially if they are 1-2 years apart. I've seen my best friend and her older sister pulling hair, ***** slapping, kicking, drowning each other, and getting each other in head locks over everything from 'who's top that is' to 'I called dibs on the front seat'.
DH is from a '2 boys 2 girls' family and there is about 2 years between each child, the older two rarely interact with him at all, and his older sister (3/4, he is 4/4) rarely interacts at all with the oldest sister (1/4). Very close friends of my family have 6 kids, the oldest is 20 and the youngest is 2, with the order being BGGBBB. The girls were the only ones who ever really fought and there is 3 years between them, but at the end of the day, the older 3 kids always help out with the younger 3, and they get along far more often than they fight.
I think it all has to do with the personalities in the family, and the way the parents bring up the kids. Its all relative (pun to be taken or left as you please :P)
