When he does things that are physical, just put him in timeout. No warnings. And when he does something bad, but not REALLY bad (like saying "no", yelling, etc) give him only one warning, saying, 'No yelling, if you yell, you go to timeout. Do you want to go to timeout?" If he does it, put him in timeout, no reaction, no discussion. It's very easy to warn and warn and warn. I used to do that... I call it armchair parenting *laughs*
I think that when he does something bad, you should be very careful not to make a huge reaction or give him that negative attention that he might be craving. If he steps on his sister's foot, very calmly tell him that he's going to timeout (almost as though you're a robot with no emotion). Take his hand (or pick him up) and very calmly, without saying anything, take him to timeout. When you set him in timeout, say again, "No stepping on sister's foot" very calmly, and walk away. When he's done with timeout, ask him why you put him in timeout (you may have to coach him in remembering). Tell him, "You're in timeout because you stepped on sister's toe. We do not step on a person's toe. If you step on a person's toe, you go to timeout." Be very careful to not show any kind of a reaction. Be very boring and blase. Then say, "All done!" Take him out of timeout, *to the offending area*, but don't let him go. Give him a hug, cuddle with him. Tell him you know he's feeling upset, but that you love him and he is a good boy. Reinforce that you love him very much.
It's very important to do this OUT of the timeout area, so that he doesn't associate timeout with getting love/attention.
Spend a bit of time with him and don't walk off right away after he offends because if he IS feeling jealous, he may not only be taking it out on the sister, but my also be trying to get attention (even if it's negative) from Mommy and Daddy. Disciplining in a calm, cool, detached manner will not only show him that he won't get the attention he seeks through doing bad things, but it will also model good behavior in the midst of anger. When we get angry and yell, etc (which i'm not saying you're doing any of these negative things, but just in case!), we show the kidlets how to get angry and yell.
I really, really agree with others about spending some time alone with him, like a little date. But it's extremely hard to have a little date when you've got a 1 year old. If this is something that is hard for you as well, then you could have a "backyard" date with him while his sister is napping. You could make sandwiches together and take them in the backyard for a special picnic together, making sure to ask him all kinds of questions about himself and how his day is going, etc. If married, when hubby gets off of work, have him watch sister for an hour while you go do something with your son once or twice a week. If he gets this little bit of time, hopefully it will help with the feelings of jealousy/neglect!
I hope that helps!