How hard to be doing this all by yourself -- sounds like you've been doing a great job, but there's understandably some sibling rivalry. A few thoughts off the top of my head:
- Could you make nursing the baby into a "special reading time" for your daughter, where you let her choose her favorite books and read them to her while cuddling all together? I only have one, but my mom said she did this with me when my brother was born and it helped.
- Some of the ideas from "Calmer, Easier, Happier Parenting" (a book I've been reading and loving) might be really helpful here. Since you might not want to buy another book right now (or have the time to read it even if you did), here are some highlights -- you're probably doing some of this already, but it could be helpful.
1. Try to do as much descriptive praise for your daughter as you can -- the book says that it's important to distinguish between nonspecific superlatives like "Good job!" or "That's fantastic!" -- kids start to tune that stuff out. What will be more important to your daughter is to hear exactly what she's doing right or okay -- things like "I noticed you played quietly while I made lunch. That really shows independence." I would think about what qualities you want to encourage in her -- given her age, and your needs, I would think focusing on her role as the helpful, independent, kind, responsible big sister would be really important. So praise anything, even the tiniest steps, in this direction, even if what you're praising is just the absence of a misbehavior. "You're not slamming the door," etc.
2. The book also talks about the importance of "reflective listening" -- helping your daughter understand her emotions better by responding to tantrums or defiance or misbehavior with a guess about why she might be feeling the way she is. "You might be thinking that I'm not helping you put on your shoes because I don't care." "You're probably feeling angry that you have to wait until I'm finished changing your brother's diaper before I can play with you."
3. Special Time. Maybe after the baby goes down for the night, you and your daughter can do 15 minutes of what you tell her is your Special Mother-Daughter Time. Whatever she most loves -- reading stories, playing a game, etc.
4. Other "big girl only" stuff -- can she help you make dinner while the baby's in his high chair or wherever? If she can gather ingredients, help stir a bowl, etc. -- and get lots and lots of descriptive praise for it -- that might help. Same with anything where she's helping you by being independent. Basically the more she realizes it's FAR easier to get praise for doing the right thing than it is to get your attention for doing the wrong thing, the less she will feel the need to misbehave.
5. Do "think-throughs" with her for how you want her to act/what the rules are/etc. At a neutral time, take about one minute to ask her some questions. Let's say you want to make a rule that when you and the baby are sleeping, she can play quietly or cuddle, but she can't yell or slam doors etc. At a neutral time -- not naptime, and not right after misbehavior -- say you want to talk to her about naptime. Tell her the new rule, and maybe do some reflective listening ("You might not like this new rule...") and praise her too, whether it's for not yelling/running away/etc. while you talk to her. You know her best. Then, the key part of a "think through" is to ask her some questions so that you make sure she understands and builds a mental picture of herself doing the new thing. Things like, "At naptime, where should you be?" "Who can touch the baby when he's asleep?" "What should you do when mama and your brother are asleep?" Don't let her say "I don't know" -- make her take a guess, and praise her for her right answers, and help her along if she's only partially right. But keep it short, and do them frequently throughout the day, and keep doing them for any new rule or behavior you want to work on.
Hope some of this is in a helpful direction... I'm vastly oversimplifying what the book goes through in terms of step-by-step ways to make these ideas work, but hope it gives you some ideas. Check out http://www.calmerparenting.com/ for more if this seems helpful.