Congrats on the new baby! I'm sure you're exhausted and this "help" from your daughter just frays your last nerve.
Rules don't damage a relationship. They set important boundaries. Tell her she hd to do this with the 2 year old (even if she didn't), and she needs to do it again. Tell her things take longer if she interrupts the feeding and you'll have LESS time for her, not more.
If you can get her to help you - pillow under your arm for nursing/feeding, getting a diaper, reading a story, making a picture - as others have suggested - great. And get some help in there - a neighbor, the father, anyone, to take her off your hands. Have someone else make lunches for the older kids, do laundry, and let you sleep. Once school is out, you may be able to get a mother's helper (even a 10 or 11 year old) to help with some activities and play (you're there for adult supervision) - and it's a good way to train a future babysitter by starting them under your watchful eye.
Yes, the novelty will wear off and the grossness of poopy diapers will set tin, but you can help it along by bringing in reinforcements.