I found when pregnant with my first that my breasts did not grow at all until my milk came in, either. I nursed him until almost 2, when I was about 6 months pregnant with my second. They seemed to then get fuller with pregnancy, then even bigger while nursing! I'm guessing that the body already knows how to prepare, like you said. BTW, I am pregnant for a third time and still nursing #2. I PRAY that my breasts don't get any bigger this time!!! But we'll see!
I found that I was still sore in those first couple of days when #2 was born, but for me it was not as bad as with #1. Right now (nursing while pregnant) is actually much more painful than with a newborn....But it could be different for everyone. I was only sore a few days with each.
My son self-weaned and was nearly two, so when his sister came, he was totally cool. He would just say, "The baby nurse your breast?" to me, I guess just to confirm what he thought. I would read to him while nursing, and I'll admit that this is when I found Noggin to be my friend. Yes, while we got used to the change in our family, my son watched his share of Noggin, but mostly just played. He still gets a little impatient when I can't immediately get up to get him some water (or something) if I'm nursing, but he's over it now. And my son always nursed at least 30-45 minutes, while my daughter is much more efficient, and even as a newborn often only nursed for 15 minutes total. Each baby is different. Like a pp said, don't worry about that until the time comes to worry!!!
Good luck!