T., I understand completely what you are going through. My son started acting out a lot when his baby sister came along, too. He was a middle child and wanted more attention. 3 rs. later, he still does the same thing sometimes, LOL.
I don't recommend you keeping him out of Pre-K though. I was just having a conversation with a kindergarten teacher this weekend about children readiness and how difficult it is to prepare children when they have not been in pre-k. Pre-K is what Kindergarten use to be a long time ago. No more is it naps and playing. They are actually learning letters and sounds, color recognition and numbers. These are things they should know before going into Kindergarten now. Buy the time they leave Kindergarten they are reading. Kids who have already been in a learning environment have a harder time picking up these things as quickly as those that have.
The GA pre-K program has excellent programs for children and its FREE. And for you, I think that the biggest benefit would be that he gets constant interaction with other children and learns structure, if nothing else.
The most difficult part for Kindergarten teachers is teaching children who have never been in a learning environment to sit still in the beginning of the year. It takes away valuable learning time from the class. So if you don't want him to go to Pre-K because YOU are not ready, you should reconsider. HE may very well be ready to take that plunge into education. It will teach him a lot and may very well take some pressure off of you for a few hours. He'll come home more content and less willing to pick on his baby sister.