T.J.
I feel for you and know what you are going through:) I have a 5 y.o., a 1 y.o. and am expecting my third the end of July (a big surprise). My best advice is to read Naomi Aldort's "Raising our children, raising ourselves". See if you library has it or you can order it online from amazon.com. It is worth its weight in gold:)
Basically when a child acts out like that it is a cry for attention. Naomi says to have 15 minutes a day at the same time every day for each child to have "mommy time" where she can pick whatever she wants to do with you and you give her your 100% undivided attention- no phone calls, no computer, no tv, no conversations with your husband etc. and what ever she wants to do (as long as it is safe and not against your house rules) goes without correcting her on how it is 'supposed' to be done. This sounds pretty easy but it is actually takes time to get used to giving a child 100% of your attention even for 15 minutes.
Also, she could be nervous about the upcoming arrival even though you are including her already. Ask her how she is feeling and don't try to "fix things" just be a sounding board for her to express her feelings. So if you ask her how she is feeling about her baby sister's arrival, just repeat whatever answer she gives such as "so what I am hearing you say is that you are nervous because you are not sure what to expect" or whatever- without trying to say how to fix it. Naomi goes into great detail about stuff like this.
When we were expecting our 2nd, we invloved my son with picking out a toy or something little each time we went to the store for his brother's arrival and that really helped him feel included and important. We also talked about how much time and work goes into taking care of a baby and what ways he would be able to help when he arrived. We got a portable diaper carrier so when it was time to change his brother, he could bring it over to "help" even though it would have been easier to do it myself, it was priceless to see how excited he was to feel included and helpful.
congratulations and good luck!