Out of curiosity I Googled it and it looks as if they are very firm on this rule - some parents reported that they had a child just a few days shy of their third birthday, called to speak to someone over the phone, and were still turned away.
Some alternate suggestions I saw were letting a younger child pick out a dress from the Castle Couture shop; taking them to have their hair styled at Harmony Barber Shop; or having special time giving them a "Mommy and Me" makeover that morning before leaving for the parks. You could also leave a gift with a note "from the princesses" in your hotel room, with the princesses saying they can't wait to see her when she's older or something like that.
I think the theme of being "too little" for various things at Disney can be a frustrating one for precocious / adventurous children... she will likely come across this with some of the rides as well. Maybe an idea would be to sit down and talk about how it's hard when you're "too little" before your vacation, but plan a few special mom-daughter things that "only" two year olds are allowed to do. If her sister is a good sport and kinda understands the idea, you could even have her join in and say "Aw man! I wish I could do that! But this activity is only for two-year-olds, no six-year-olds allowed!". Maybe the two of you go and get a dessert together, meet a particular princess together, something like that.
