What's with twins/female duos in pop music?

I don't think it has anything to do with record labels. If you think about it, The Donnas have been together since 1993, Tegan And Sara have been a band since 1995, and Meg & Dia (my favorite band) were in Coward's Courage and Jade Harbor together in their teens before they got signed as Meg & Dia. And you can't forget The Watson Twins, they've been making music since 1998, and they used to be known as Black Swan before they released that album with Jenny Lewis from Rilo Kiley. =]