I always feel so guilty when I add a new kid to the mix. I am worried how the youngest will handle it. When I went from one child to two I felt so badly that I was consumed with the care my newborn required. Plus my 2nd child was colicky and very hard to appease so I concerned that my oldest would feel neglected or that Mommy didn't have time for him. He did occasionally feel like this and it took some adjustment but after realizing that I needed to carve out special one on one time with him things got better.
My 2nd child in October went from the youngest to the middle child with the addition of our 3rd and final child a little girl. My boy was used to getting lots of hugs and cuddles throughout the pregnancy. I was so sick with my baby girl's pregnancy I couldn't do anything else. Then when I went to the hospital to give birth we were there 5 days. This did not help with his new big brother status. When I arrived home he was so angry at me for being away so long and was not crazy about the little baby girl I brought home.
Over time, he has started to accept having a little sister especially when she started to react to him with smiles, giggles, and noises. Fortunately, when I go out with all 3 kids by myself my oldest takes care of his little brother and watches out for him. Now sometimes they wind up amping each other up when we are out together but overall it goes ok.
We have found a rhythm now and things are going pretty good. Since mommy is busier it teaches the older kids to be more independent which is a really great thing.