Arrange special treats or special events that aren't appropriate for babies, but that your older child can enjoy. At least occasionally offer special foods, or special outings, like a trip to the movies or an indoor playspace. If you make a point of noting that your baby can't enjoy these pleasures yet, your child may appreciate being more grown up. (As a bonus, this may also relieve some of your child's anger toward the baby, replacing it with pity for the poor miserable thing.)
Regression in certain abilities—or in the willingness to perform up to capabilities—is common among both toddlers and preschoolers when a new baby arrives. Just when you most need your child to dress herself or feed herself, she suddenly won't do it. The reasons are easy to understand. Consciously or unconsciously, your child decides that if she were a little baby, too, you'd pay more attention to her. So no matter how desperately she tried to prove that she was a big girl before the baby arrived, your child doesn't want to be a big girl anymore.
Try to handle regression with grace and good humor. Let it slide as much as you possibly can. Of course she doesn't need to act like a baby to win your affection. But what's obvious to you might not be so obvious to your child. So shower her with praise whenever your child helps out in any way—whether that's helping to care for the baby or helping to care for herself.
But at the same time, let your child know through both words and actions that she doesn't need to push herself to be grown up either. Avoid the odious phrase, "Act your age!" Make it clear to your child that you always will love her, no matter how she behaves.
Play along with your child when she plays at being a baby. Transform it into a game of make-believe that allows you to baby your older child a little. If she suddenly talks in baby talk, coo to her the way you do to the baby. If she crawls around on the floor, scoop her up and give her a big hug. If she insists, let her drink from a bottle for a while.
It's important to your child to know that whatever you give to the baby you'll give to her if that's what she wants. Let her know that the reason you stopped feeding her and dressing her and bathing her in the sink before the baby arrived was not that you no longer wanted to do these things, but that she had outgrown them. In taking this kind of approach, you give your child the power to make a choice about how to behave. If she knows that she can freely behave like a baby or get what the baby gets whenever she wants, chances are she'll soon stop doing it.
While allowing her to act like a baby, help your child recognize the advantages of being a big kid, too. Give her some new privileges and, if she's willing, offer her some new responsibilities. For instance, you may make a point of letting her stay up an hour or so after the baby goes to bed at night.