I agree with everything that's been said. It's hard being an older kid - I remember it well when my brother was born.
KD is jealous of Cassi and It really started when Cassi started to become more of a "little girl" and less a baby. We found ourselves praising her for talking, counting, singing, and becoming potty trained and I think Kennedy felt left out. I find I have to give special time to my oldest every so often and she really appreciates it.