J.R.
Yes, mine is going to be another post that is long on sympathies and short on advice. But I'm gonna write it anyway, just 'cuz sometimes it feels good to get an "Amen!"
My oldest daughter (McKenna) turned four at the end of October, and my youngest (Chappell) just celebrated her first birthday a week ago, so our age spans are nearly identical. Ditto on the behavior problems, but recently, I have seen an improvement. A couple of thoughts I have on what things might have helped:
1) We are teaching Chappell Baby Signs, something we did with McKenna too. McKenna loves helping her baby sister learn new signs, and it has made her realize that she can communicate with her. It has also helped us exhibit to McKenna that in our house, when you are able to communicate, you don't get what you want by screaming. We encourage Chappell to use her signs to tell us what she needs, instead of throwing tantrums, and it has helped to reinforce the point with big sis.
2) I resist the urge AT ALL COSTS to compare the two. Several times a day I find myself wanting to say to McKenna, "Chappell is a baby and is behaving better than you," but I don't want to amplify the animosity that already exists.
3) I have tried to have conversations with McKenna when she is not in trouble for any reason, in fact when she is being a really good girl. I start by praising her for being sweet, tell her how much I LOVE being with her when she acts that way, and on the flip side, how she is not fun to be with when she is tantruming. She gets it, and now thanks me for taking her to the park, etc, and I think tries harder to make me want to do fun things with her.
4) We have tried really hard recently to give her an opportunity to "turn back from the dark side," as my husband and I refer to it. In the past, when McKenna would act out, my husband and I would get angry, and start responding in kind with whatever punishments we could think of at the time. She would then "go for broke," and just get worse and worse. Recently when she starts to act out, we call her on it, tell her that she has the ability to get it under control. We tell her what the consequences are if she doesn't get it under control (time-out, lose priveleges, etc), and she has a choice to make. We are very intentional about saying, "You can either choose to be a good girl and continue to play, or you can choose to behave badly and go to time-out. It's your choice. What are you going to do?" McKenna seems to have really responded to the concept of choices. I'm trying to give her other opportunities to be in control as well, by choosing what she wants to wear, eat, what game we play, etc. I think it gives her a sense of personal resposibility.
Now, McKenna is definitely not perfect and still has some meltdowns, but they are much less frequent than they were. I wish I had a magic solution for you (and myself), but no one ever said parenthood was for wimps!! Good luck, and let us know how it goes! Please write back if you find other solutions that you think might help us too!