First of all, I think it needs to start really early. I let big sis feel little sis moving around in my belly and answered any questions that she had (they have a two year age gap). We talked about how to treat and care for babies using big sisters dolls before the baby arrived.
When our second was born, we didn't stay at the hospital very long so we got to introduce them at home, which was really special. We let big sis sit on the queen sized bed and snuggle little sis with us right there. We let big sis be involved in the special moments like little sis's first bath. We put the baby bath on the kitchen table and let big sis use the wash cloth. We had lots of dolls that were literally the same size as the new baby and I made big sis her own carrier while I used mine. I let big sis use the breastfeeding pillow and "play mom" with her dolls. They had baths together with the baby bath fitting in the tub. We still did lots of the same activities as before the baby, like toddler art setups. We actually had the bassinet on the main floor during the day so that little sis would be near us and would get used to sleeping with some noise. Im pretty sure it worked, she is an amazing sleeper as a 6 year old. We did lots of tummy time for little sis while I got down to their level and taught them to play together.
How did I teach them to play together? I talked to big sis about her little sisters development when it came up. For example "oh, she is learning to build when she knocks the tower over. Do you think we could make a tower for her to knock over?" Or she's not telling us words yet but what do you think she could be feeling? I always tried to narrate what I thought little sis was trying to tell us as they day went along, sharing what I thought little sis needed.
