I have a 3 year old daughter and no plans to have any more kids (more because of medical reasons than anything else). Honestly, my take on this is that kids are kids. They all have their own personalities. Sure there are differences between boys and girls, but your 2 girls could be totally different from each other in terms of personality and temperment. I do have 2 stepsons that are only a year apart in age (now 16 and 17) and they are like night and day. Both boys, but as different as oil and water.
My advice would be to recognize that, as they grow, they are individuals and need to be respected as such. Do not expect the older one to always "set an example" for her younger sister and do not allow the younger one to act the baby just because she is younger. My stepsons' mother tended to do this and it seemed to me that it just created more conflict and sibling rivalry. She would also constantly make comparisons in front of them and talk about them like they weren't there and could not hear: "He is more athletic and his brother is more artistic, etc.". Later I read somewhere that this is one of the worse things you could do in terms of siblings learning to get along. It was this constant dynamic, in terms of the younger one acting the fool to get attention, being a pest to the older one, older one getting ticked off and yelling at him, then mom telling older brother to be nice to younger brother because "he's younger than you, he doesn't understand, etc. " I wanted to say, hey, he's not that much younger - it is only a year's difference!
But I digress...my point is, don't cast them in roles...recognize them for the individuals that they will be and embrace it. Encourage the both of them to be strong self-confident girls who find their talents and their strengths and learn to use them to their full advantage.