C.C.
My older daughter was 3 when her little sister was born. We had a VERY hard time with the first few months. On the one hand, my daughter seemed fascinated by her baby sister, but on the other hand, she felt jealous of her and was clearly resentful despite our efforts to spend one on one time with her, etc.
Someone got me the book Siblings Without Rivalry, and the one thing that really hit home was that bringing home a new baby and expecting your child to immediately love this new baby is a little like if your husband were to bring home a cute new wife, and telling you that he still loves you but isn't this new wife cute? And in time, you will grow to love this new wife! And be nice to the new wife because she can't defend herself... and everyone who comes over only wants to look at and talk to the new wife... WELL, that pretty much said it all to me! I had never thought of it that way, but clearly this was how my older daughter was feeling. Even though she knew that she SHOULD love the new baby, she just didn't, because children aren't programmed to want to share their parents' love and attention at that age. And from her perspective, the whole thing was a bad deal for her and she didn't see any reason to change that perspective.
Obviously there's no way to change the older child's perspective on this. It is what it is. But if you acknowledge their feelings ("Are you feeling a little sad because I have to spend so much time with Baby, feeding him and changing him and putting him to bed?"), and maybe talk about how it was when your daughter was a baby, especially if you can show her pictures ("Here's a picture of you when you were Baby's age - see how cute you were? I am so glad that you are a big girl now, because now we can have fun together at the park, and I love how you help me when we cook dinner too!") Just acknowledge that although she is not the new baby anymore, it's okay because she is growing up, and that's a good thing too. Over time as your son gets older, he will be more fun and she'll like him better.
Your post brought to mind that when my youngest was 3 months old, she was sitting in a bouncy seat and my older daughter was singing to her. I thought, oh, how sweet! And then I listened to the song she was singing. The lyrics went, "I'm the first one, I'm the best one, and you're the stinky baby, the stinky stinky baby!" And my little one was in the bouncy seat, kicking and smiling, just thrilled that big sister was singing to her! Makes me laugh to this day!