Katie,
Try to encourage Abby to be a big girl - use her words, be Mommy's helper, etc. and then fuss over HER when she does the right thing. Have Daddy join in the act too. When he comes home, let him greet her first. Then tell him how GREAT she was - how she helped and things she did to act like a big girl.
When the baby does nap, make sure that you tell Abby that this is BIG GIRL time. SHE gets to pick a game or activity. Even if you need help doing something simple like emptying the dishwasher or sorting laundry engage her and tell her what a great job she is doing.
Make sure she understands that BIG GIRLS get big girl privileges like PICKING her drink, her snack and her games to play. BABIES don't. Babies who fuss and need Mommy's help only get to eat, sleep, poop and nap.
This helped our Big Girl to understand that helping Mommy and acting like a big girl has some great benefits. It especially helped that Daddy made such a big deal out of how proud he was that someone was at home to "help Mommy" get diapers, binkies, etc.
You are absolutely right that her crying and screaming is an attention seeking behavior. However, this is a very hard adjustment for her to make and she may not yet understand what she is feeling or how to react. Try to stay composed and remind her that she needs to "use her words" and a nice voice or you will not talk with her. If she screams or becomes angry, I would sit her down and tell her "when you are done having your 'moment' you can come talk with me." It's not so much a "timeout" as in punishment, but a cooling down time for her to get composed and then she's free to get up to come talk with you. (I still do this with both of my girls when they are tired, cranky or just not using nice words.)
Finally, someone suggested to me something invaluable...take a moment every so often to tell the BABY, "Baby, you're going to have to wait. Abby and I are doing something right now." Of course, the baby doesn't get it, but ABBY DOES! She sees that you are not always rushing to help the baby. You are telling the baby to wait her turn. You are making Abby feel important. That way, when you have to ask Abby to "wait her turn", she will begin to understand that it's a TURN, not constant attention on the baby.
My final word of advice, does Abby have a newborn baby doll of her own? We got one when I was pregnant to "teach" our older daughter how to be nice to the baby. I would suggest one that is ALL HARD PLASTIC - that way it can go in the tub, get lotion put on it, get diapered, etc. just like Baby sister. My daughter loved playing "Mommy" to "Lula". She mimicked everything I did, down to lifting up her shirt to nurse Lula. It was too cute.
Best of luck - let us know how it goes.
Sara