Dressing siblings of the same sex with the same clothes is an Argentinean custom that Maxima obviously enforces. This ends when the siblings reach age 12 or 13 and are considered wise enough to make wardrobe choices.
Thanks for the info,
Grand Duchess.

I had no idea that was an Argentinean custom. I always love learning something new!
Still, 12 or 13? Yikes. My mom loves to tell me the story of how we would have fights every morning when I was 2, because I wanted to wear something other than what she picked out for me. This continued for a brief period, until she asked herself, "Why am I arguing with a 2 year old?" Needless to say, after that, I wore what she picked out for me.

I definitely think 2 is too young for a child to pick out his or her clothes, but 6 or 7 seems more than reasonable, as long as the child is picking clothes that are appropriate for the occasion/weather.
I do agree, though, that the girls always look well-groomed, loved, and most importantly, happy. IMO, that's what distinguishes them and the Norwegian, Spanish, Belgian children from their poor Danish counterparts.