We had no problems - typical "lazy house model"! We just made sure that she had "her" time after Jake went down for his night time sleep and she has been fine ever since. He is 2.5 and she thumps him when he gors too far but has never tried to murder him for usurping her position. We have just brought home number 2 and she hasn't flickered a whisker. She comes and curls up next to me in bed when feeding DD and then DD does back to her bassinet, Millie crawls under the blankets and has her cuddle till next feed.

I guess if you chucked them suddenly outside you could encounter problems, but we just treated it like introducing a new baby to a toddler when DS1 came home.