We've been dealing with this too...Ava knows I can't easily jump up while nursing the baby and that I'm tired and distracted. She capitalizes on this by misbehaving or just doing what she wants during those times. She's *never* been irritable or mean to the baby though, just me.
I'm also trying to give her special time (which honestly is hard to do, especially just us with dh working so much overtime) and lots of kudos when she does a good job or is particularly helpful. I've also worked at giving her special things to do that only "big girls" get to do. She's had a lot of summer camp activities, time with grandma and "helping" daddy with projects and keeping her busy with fun things really helps in the long run.
Good luck mama, this one is hard!