Packing - Babies
Babies are simultaneously easy and difficult to pack for. On one hand they aren't particular about their outfits and don't care if they wear the same thing everyday; their clothes take up very little room and are easy to wash. Alternatively, babies need all sorts of 'stuff' like diapers, ointments, wipes, and all sorts of other items that in this century we feel we can't live without. Can you live without the wipe warmer and diaper genie? If the answer is no then perhaps this isn't an appropriate undertaking for you and your clan. Traveling with a baby will force you to live with the bare minimum, and you may realize that perhaps all of that 'stuff' isn't as necessary as you thought it was!
Diapers - they sell them everywhere. This is testament to the fact that we are not the only society on the planet that doesn't give birth to toilet trained humans. Unless you are particularly choosy about the brand you use, I would recommend taking enough to get you through a few days and then buy them locally. If you took enough diapers to last 3 weeks you'd need a whole suitcase for them alone, which I suppose could be handy if you need the space for souvenirs on the way home.
Bring all the little comfort items that your baby (and you!) need, but try to avoid taking their entire nursery. Use the opportunity to see what other cultures use for their babies; take a trip to the local grocery store or drug store and test out new products. In the end you need to do what makes you comfortable to ensure that you all enjoy the trip.