Throwing a new sibling into the mix almost always gives older children some serious adjusting to do, and if your son resents his little sister but isn't allowed to say so or to act on it, then he could indeed be taking his frustrations out in other ways.
And the simple fact is, kids this age have not yet developed much empathy, so are just plain "raw" in their likes and dislikes. It's very honest, actually – can you think of people in your life that you don't like, but perhaps pretend you do? And can you make yourself like them just because someone tells you you should? Nope, I can't, either.
Please don't start labeling him – this creates a self-fulfilling prophecy that children all too often live up (or live down) to. Continuous punishment is almost the same thing as no punishment – it just becomes the steady-state reality for that child, and he comes to see himself as hopelessly bad.
I'd try a new strategy. At home, I would role-play the situation, with stuffed animals for puppets. Let your son play the part of the aggressor, and have the other animal(s) ask him what he's doing, and why. He probably won't be able to say much, but it will help him puzzle out his feelings.
Have a big animal come and talk to him, ask if he needs to be separated from the group. No? Then what other action does he choose so he can keep on playing? (Maybe apologizing, sharing, or taking out aggressive feelings on some inanimate object.)
Yes? Then, does he want to sit over there until he settles down, or should we all go home? Positive choices, rather than unbending punishment, will give him a chance for a redo, and the possibility of actually practicing alternative outcomes. That just can't happen with end-to-end time-outs.
Finally, switch roles. You'll be surprised at how well he'll be able to play the parent. He already knows by heart what parents tell him. The piece that's missing is his emotional development, and that will need tender coaching to help him bring out his best. So try to get him to play the role of the creative and sympathetic parent, and pay attention to what he reveals – it will be what he perceives he needs from you when he's acting out. (If it turns out he can only scold and punish, that's because that's all he's had modeled.)
You can also use this approach to gauge his reaction to his little sister. And help him find ways to express his almost certain frustration and disappointment with having to move over and make room for a baby.
Role playing with puppets is a fabulous way to look inside our children's minds, because they don't know how to identify and report very much of what they're feeling. But all behaviors are strategies to try to meet needs. Little kids just don't come up with very good strategies yet. They need opportunities to find alternatives that work better.
Kids crave our attention and approval above almost all else. If they feel the approval isn't there, they'll still seek attention, even if it has to be negative. So "catch" your son doing acceptable things all day long. Notice when he tries to make his sister laugh. Comment when he comes to the table on time, or picks up his toys. Remark on his attempts to play peacefully or constructively beside his playdate friend. Tell him how much you like his smile.
I'm sure you try to do this. And you're busy and probably stretched far too thin most hours. But if you can train yourself to give him regular, positive feedback, he will probably rise to your new estimation of him.