G.B.
I don't think this is totally surprising. He's at the age where he's trying to figure out a lot of stuff and there's this cute baby in the middle of his world. I'd put firm boundaries where you need to but always show a lot of empathy & understanding for him in this. Get him to talk when you can. Maybe you can even get him to play-act his feelings with stuffed animals or action figures. He has some fears about her because he doesn't realize you can love both of them enough and that he won't be losing anything but actually gaining from her being there. It's very hard for kids this age to understand sharing. Let him have some special things he doesn't have to share and put them where the baby and other children can't get to them so he knows he can control that little area. Emphasize both the big boy aspect of his age as well as the still being a baby in some ways aspect. Cuddle him and rock him and whatever to fill his needs. I have a 3 yr old granddaugther and I swear - sometimes she's so grown up and sometimes she's still a baby! This is how it is at this age. The best thing in the world for him is that he has to accommodate his sister and learn to protect and assist her. Tell him how much she'll look up to him if he's kind to her. He'll be her super-hero!
This will pass, A., if you handle it with gentle, loving guidance. Hang in there!