I took something my daughter said and ran with it as an opportunity to introduce the subject. Since then, I've made more mention of it, with more and more details as she asks more questions and the time draws near. We were discussing all the things she is looking forward to; sharing her bed with her broth, painting his room pink, feeding him bottles but not pizza. So the bottle but not pizza thing was just the perfect segue in to mommy makes milk for the baby. Later, that fit in perfectly when she poked me the nipples in the bathtub and asked what those are. So I just listen to her and share the game plan.
I've been able to ask some leading questions once I figured out how much she has been able to synthesize and come up with on her own. I am also being open to any natural curiosity and exploration, so if she wants to know why I have boobs and she doesn't, I'll explain. One bonus to this process has been to further the concept of sharing. Mommy makes the milk to share with the babies, and partners/children get to hang around feeding time and share their love and kisses while baby is getting the milk.\
Good luck, hoping it all works out for you!