We have a zero tolerance policy for the kids hurting each other (or us) they go immediately to a time out! no warnings or sweet talks.  
 
I do think that you need to make some time for alone time with each one of them. the older one probably feels "neglected" (for lack of better term) She used to have all the attention and now she need to share it with a little one who need a lot of attention herself.  
 
You and your husband should make a schedule that each of you spend alone time with each of the girls.

Hello,
First of all, take some time off from the kids. Maybe treat yourself to a day spa or other nice outing. You definately deserve it even if you feel you don't. What you have is Burned Out Mom Syndrome. When this happens, rarely can you be the mom you should be due to stress overload. I would also advise a mom's day out program for you as well. These years are high energy years and sometimes it wears a mom down.
Secondly, once you have had time to gain some of your time back, you can start to work on these challenges at home. I would also recommend a multi-vitamin with no extra additives. Also, ask dad to come in and help the days you feel overwhelmed.
Good Luck

There is an excellent book that I would like to recommend, Siblings Without Rivalry by Adele Faber and Elaine Mazlish.  This book clearly explains the most common reasons why older children become jealous and act out when a sibling comes along.  Also, strategies for coping and changing behaviors are spelled out in this book.  These authors also have a companion book titled, "How to Talk So Kids Will Listen & Listen So Kids Will Talk"  that may be a good one to check out, as well.
 
Good luck!

Get a cloth placemat. When she kicks the baby put her on the placemat, on the floor. Keep her there for 4  min. Ignore her for that time. If she leaves the placemat, just calmly put her back on without talking. If she starts crying or screaming, she's trying to get attention. After 4 min., tell her she has to be nice to the her sister and she can go play.

Well i would assume you are telling her that it is not appropriate to kick anybody, especially babies. And tell her it is making you angry that she is hurting your baby. And then since she does it again, i would assume you place her in a time out away from baby since she cannot handle playing nicely WITH baby. She may be spending a lot of time on her bed for a few days while she learns the negative affects of her negative actions. Ask her if she wants to be kicked every time she does something you don't like. Tell her its not an option and she needs to ask for help when she is frustrated with the baby.