Who is the ugly sister?

Ok, here's a mystery for you...
Why is it that you can take any pair of sisters and you will always find that the elder of the two is markedly better looking?
A shocking hypothesis, I know, but it's true! It is true of every pair of sisters I ever met and true of every pair of celebrity sisters you can name: think of Paris and Nicky Hilton; Jessica and Ashlee Simpson; Hilary and Haylie Duff; Jodie and Jemma Kidd; Kylie and Dannii Minogue; and the list goes (as they say) on and on.
What is that about???
Is it an inbuilt genetic fail-safe so that parents would bond with their first-born and feel safe to create more babies?
Or is it because first-born babies are far more carefully cared for, better fed, and generally more fawned upon than the siblings that follow?
Or perhaps they just exhude more confidence and therefore seem more attractive?
Any theories out there?