How do I keep my four year old daughter from trying to carry the newborn around the house?

My friends have just had their first child togther and now that the baby is home and four days old, the other children want to help. The problem is the four year old is trying to carry the neworn arond everwhere. When the parents look away for a second she is trying to pick the baby up. Worst yet they live in an upstairs apartment with exposed stairs. Any ideas on how to keep her from trying to be a "big girl" and carrying the newborn.