3Yr Old Behavior Problems

She is getting some sort of satisfaction out of that action, or it wouldnt be continuing. Now that she is hurting your younger daughter you really have to step in and put a stop to it. Have you talked with your pediatrician about this...to find out if there is a physical reason for this behavior?? I would think that would be a good place to start.
I know it is hard...but try to stay calm as you react to your daughters actions...dont let her see you raising your voice or getting upset...give her a good example to follow. Calmly remove her from the situation...put her somewhere for a "time out" (Maybe use Super Nannys idea of one minute time out per year of age)...talk with her a little about why her actions arent something that you can allow. I wish I had more solid advice for you, it is so individual...what works with one child doesnt work with another. I really think your pediatrician is a good resource...he/she knows your children and has dealt with things like this hundreds of times before.
Good luck...just stay calm and stay consistent...
R. Ann

i'm probably off, but could she possibly have sensory integration issues? my now 7 year old has sensory integration dysfunction ** http://en.wikipedia.org/wiki/Sensory_Integration_Dysfunction ** (diagnosed at age 3) as well as aspergers syndrome (recently diagnosed and in her case is genetic) and she did all of the things you just described. some children aren't able to process or block out sounds, touch, sight as well as most people and it becomes overwhelming to the point where they don't know what to do, so they lash out at those around them. think of how you feel when you're overwhelmed, you get irritable. the same is true for these children. take note of her surroundings when these outbursts take place. is there a lot of noise from different sources, is she in clothing that is uncomfortable to her, is the tv on in addition to perhaps a radio or your two year old playing or talking? with us, it was always a game trying to figure out what was setting my daughter off, ha ha. i laugh now because she's learned coping mechanisms to handle this quirk of hers. at the time, it wasn't so amusing. another could be Oppositional defiance disorder or ODD. she's also three too, which is a common defiant age,it's the aggression that caught my attention though. i apologize if she doesn't fall into these "categories" but thought it was worth a mention, rather than just labeling her as being difficult. regardless of the cause, i wish you luck!

My 4 year old has been intense since she was born. I really like Debby's tip about offering 2 choices, one she would hate to do... I'm going to try that myself.
Here are some things I've learned about my sensitive child: Try to recognize when she's going to have a melt down before she has it. One of my daughter's triggers is if she doesn't have a snack at least every 3 hours, she gets irritable (her dad is the same way). Another trigger for her is pure frustration. It comes from being inside all day, without enough physical activity. Her dad plays chase with her alot, me sometimes (I'm the sit down and do crafts or draw and color parent) so if I see that she's getting restless, which leads to frustration; I'll get the little trampoline out and have her jump for a while. Setting a timer or having her count keeps her focused long enough to wear herself out. One more thing you can have her do that will be a good exertion of excess energy and help her feel a sense of accomplishment is have her do little chores. I have my daughter pick up all of her toys in the living room, put things away, and throw away trash before she can watch tv. It gives her something to focus on and she does a lot better if she's contributed around the house. If all of that has been addressed, she needs to learn to calm herself down and you have to give her the tools to do it. I have my daughter take deep breaths, count, etc.
You can also make your directions to her less confrontational by distracting her. Talk about something silly related or unrelated to what you are having her do, all the while guiding her to the expected behaviour.
Another thing you can do is sit down and play with her 5 minutes a day, just you and her and give her nothing but praise. "I like how you're doing that, show me how you do that; You're so smart, how did you come up with that;" etc. When the 5 minutes is up, say,"you're doing such a good job there, keep it up and I'm going to go wash the dishes." It gives them a boost of recognition, she gets some one on one time with you, and it's a good positive foundation showing her respect and love which she will return.
Another very simple thing that really helps the intensity of my daughter's outbursts is to simply tell her she can cry all she wants but she has to keep her mouth closed while doing so. I learned this from a friend who has 5 kids. LOL! I wish I had known it sooner... The bellowing of her own voice would escalate her tantrums exponentially... now, her tantrums are much shorter and much less intense.
Whatever you do, be consistent. Make a plan and stick with it. It will take some persistence and discipline on your part to redirect your daughter's behaviour. If she gets to were she knows exactly what the consequences of her actions will be, she will be a lot more cooperative. If she has some reward to look foward to for good behaviour, say a ride on those quarter machines at the grocery store; that can be a good tool as well.
My daughter and I but heads alot. I just quit my job, so I'm home with her now, where my husband used to be. He is very laid back with her, so she was used to pretty much getting her way. I, on the other hand, am just as stubborn as she is. Get your husband on board with whatever you decide to do, this will help the consistency factor. It's been a struggle for us, since my husband is pretty much happy to let her do whatever she wants as long as she's not hurting herself, but I'm working on him.
It's funny, before I had my daughter, I thought of children as empty slates that you can write upon and if you give them love, structure, and boundaries, they'll come out as close to perfect as possible. But little did I know how much personality would be instilled before she even came out or how difficult it would sometimes be to maintain consistency despite my own human error.
You might ask yourself, am I empathizing with her? Being a child can be very frustrating. I find myself sometimes getting caught up in my own agenda and barking commands at my daughter, which I don't think anyone responds very well to. When you get to that point, you have to step back and question what you are doing.
I know your frustration. Hang in there. That stubborn personality, if directed right is going to make for a strong, independent women someday (hopefully). =)

my 3yr old went through the same thing. there was about 2 months where she did nothing but scream and throw fits. just remember to stay calm dont let her see (or senses) that you are frustrated. stay consistent. you might also try taking away stuff. all you are required to provide is clothes a bed and food. take everything out of her room but her bed and tell her she has to earn everything back. i also found it was helpful to teach my 3yr old how to take deep breaths and count to ten (sometimes we have to go to 20) and once she calms down we said "Thank you for calming yourself down". even if she does calm down, you still have to follow through with the punishment. it took a while to get my daughter throw that phase but know she is happy and she remembers to count and take deep breaths when she is getting upset. hang in there. it is tough but i have been in the same position. Prayer also works wonders!

As the other's said this is probably a phase and being as emotionless as possible is best.

Here's a good guideline of why behaviors happen: http://www.lifematters.com/step.asp and to handle it you offer choices like, you can leave your sister alone or you can go to bed. Pick something she absolutely hates for that 2nd choice. This way she learns that her actions are her choice and she has to either reap the rewards or the punishment. The first choice you offer should be some redirection or following an instruction like take that toy back to your room.

I want to make sure you are spending time with her independently of her sister and step brother. I wrote out 2 emails that I moved to blogs and I'll post the links to both of them. I wand to preface the links with how we achieve this independent time. My husband does the bath and bedtime routine. He starts the night 45 minutes before the first bed time this is when the kids are expected to pick up all the toys around the house and put them away. Then he starts the youngest one's bath half an hour before bed time and at 20 minutes before bed time he sits down to read to him. This gives Tiger 20 minutes of reading each night. I have worked in "special time" with him as well just before I put him to bed. While my husband is putting the older child in the bath.

you have to work on this one first and when you notice that it's also in your daily vocabulary then move on to the other one
http://debbysewninwesttulsa.blogspot.com/2007/06/parent-c...

http://debbysewninwesttulsa.blogspot.com/2007/06/pcit-par...

For right now see if the Claremore Library can get you a copy of The Explosive Child by Ross Greene. It helps to identify those situations where she's going to throw the tempertantrum and how to handle it.