Hi C.,
I can empathize with you! My daughter just turned five, but she went through the terrible two's and three's, and a year ago for about4 months before she turned four, she was just as obnoxious as could be- very strong-willed, talked back, wouldn't listen, was a bit out of control herself. I didn't know what to do. We had tried everything, as you have, and we were consistent with it, but she was very trying! I too had my son who was a baby at the time and she definitely got worse after he was born.
A friend of mine who also had a daughter like this told me a great tip to try, in addition to everything else. What she told me to do was to make a good behavior chart, and for every good thing she did, give her a sticker and let her put it on her chart. In the beginning- you compliment anything she did that was good- for example- "you were nice to your sister/brother"- she gets a sticker; "you finished your milk", "you didn't throw/hit/spit", "you got your shirt on all by yourself"- at first you find anything that is good, no matter how small or trivial it may seem, and they get the sticker. Once she earns the sticker, you cannot take it away- she doesn't lose it. If she misbehaves, you stick with the timeout, or taking a toy away, or whatever, but then you can remind them of the good behavior chart. I told her that if she got like 35 stickers by the end of the week, she would get a prize- and you can make that whatever you want- give her a nickel for her piggy bank, or whatever you want. so I always made sure to find at least 5 or 6 things to compiment her on each day, so she'd have enough stickers by the end of the week. You could tell her that if she gets so any stickers by the end of the week, or has 2, 3, or 4 straight weeks, she will get to go on a mommy and daughter "date"- whatever you think will work for her.
Anyway, the purpose of this is to start encouraging the positive behavior. Plus, it also helped me to get out of the rut of only seeing all the bad things she was doing, and to focus on the good things she was doing. It worked for me! Within a day or two, she was excited to get the stickers, and started to behave better. And, as her behavior started to improve, I didn't compliment EVERY thing she did, but you have to in the beginning. Like I said, we still did timeouts. We also had "the Tuesday box" (you can pick any day of the week). And, if she wasn't behaving, then a favorite toy would disappear into the "Tuesday box" which meant she would not get to have it back until the following Tuesday. We would always give a warning first, and tell her that if she continued, something would go into the Tuesday box. After awhile it caught on.
Anyway, I hope this helps. Like I said, it worked for me. Once my daughter turned 4 she became a very sweet girl again. So there is hope! :)