I wouldn't wait to do something either. You don't want your child to develop these bad social habits. I would suggest reading the book by Jim Fay called Parenting with Love and Logic. It is a great resource that really helps in these kind of situations. While your daughter may not understand why she is behaving this way, it sounds like she is able to understand directions. This book is all about offering choices to give the child some ownership in their own life. You decide the choices that are ok with you and she gets to choose. This way you have set boundaries that are acceptable no matter which choice she makes. (example "would you like to to play with your toy or watch a movie while I feed the baby?")
You could even talk to her about what it should look like/sound like when you are attending to her brother or putting her to bed. See if she can tell you in her own words. This will let you know if she truly can follow directions or not.
Set up some kind of reward system (i.e. sticker chart/behavior chart- you can check at local teacher stores for these kind of things)When she does something correctly reward and praise her for it. Do this often at first (after each positive thing she does and then after she follows a couple directions correctly, etc.) Also make sure when praising her to connect it to an intrinsic motivator such as "I really liked the way you watched your movie while I fed baby brother. You are such a good helper for mom. Doesn't that make you feel good inside?"
This way she will start to want to please you instead of getting a sticker, reward etc.
If she doesn't follow through with your direction or choice she made, then you will need to set up some sort of consequence such as time out. At her young age, she shouldn't be in time out for more than a few minutes. Set the kitchen timer and then ask if she is ready to play nice again. Make sure you tell her what it was that she did wrong. This is the hardest part. You have to be very consistent with your actions so she knows you mean business.
I would also plan to do something special with her (just the two of you and your husband should spend some one on one time with her as well) several times a week if not daily. It sounds like she is acting out for attention, so you need to focus on giving her some alone time, but not at the expense of turning into a holy terror for you.
I also highly recommend the show Nanny 911. They provide a lot of good strategies to set up structured routines that are fair and easy for kids to understand.
Good luck to you! I am expecting a daughter in May and my son will be right at 18 months old. I may be in the same boat! Ugg!!