My guess is that yor are correct, it is a jealousy issue!
Obviously when you have a child your attention is now divided a little between Rocker and your new daughter. Rocker didn't like that, and can't understand why your aren't paying attention to him as much as you used to and it's only going to escalate with your second child. This happens a lot with Cockatoos and many of them get rehomed for this issue.
How do we fix it, well this is a very "slippery slope" and as Meghan has suggested positive reinforcement IS going to be your best option! Rewards and attention when he is quiet and ignoring the bad behavior. You need to think of Rocker in the same way that you think of your three-year old. When she does/says something that you do not want to reinforce you ignore it, when she's doing things that you want to (like potty training) you praise and reward lavishly. The one thing that you must be extremely careful of, is just like a three-year old, negative attention ("Rocker, shut the %#*&$%^*@&^# UP!") is still attention.
Another thing that might help is to establish a predictable routine for the entire family. For example my girls (Hamlet and Mac) know that in the morning they are going to get cuddle time and hang out time with either my wife or myself or even both depending on the day, and they are also going to have shower time as well (if I haven't taken a show by like 11 am because I am being very lazy they start to get antsy and after the shower they calm right down). Establish a routine where Rocker gets some one-on-one time with you (maybe while the children are napping) and this might also help with the yelling.
Also please make sure that you have lots of interesting toys, foraging and other wise, in his cage, this will help to distract him a little when you aren't paying attention to him.
That's all I can think of, but I am sure that our resident "animal behavior" expert Auggie's Dad can chime in here with more that maybe I have over looked/not thought of.
Welcome to the forum, and congrats on the new baby ~ my wife and I are expecting our first around Halloween. It's very exciting!