Rolling Stone On Olsen Twins

The Rolling Stone cover story on Mary Kate and Ashley (“America’s Favorite Fantasy” it says. What the heck? Sickos) is actually kinda interesting. Anyway, they’re fraternal twins, not identical. But why is it that same gender fraternal twins usually look really similar? They should look as much like their other siblings as they do each other, right? But the fraternal twins I know all look really really similar. Odd.