D.D.
You actually answered your own question. She'll poop on the potty if she doesn't have a pull up on. My granddaughter is the same age and wears underwear only. No pull ups even if they are going to be running errands. Let's face it; pulls up are just diapers with a different name (and more expensive). She's 3 1/2 and has no need for diapers any more.
Make a big deal out of the stuff she can do that the baby can't. Poor baby can't use the potty. Poor baby can't use a fork to eat. Poor baby doesn't know her abcs or how to sing songs. If you refocus your daughter on all the wonderful things she can do that they baby is too little to know she'll feel proud of herself. Push all the positive reinforcement of her skills should help your daughter.