Thank you for the additional information, often times if we can take a step back and view it as not part of the family it is easier to see what the problem is and the best route to take.
When talking about preteens we tend to label it as behavior modification. We do not want to try a battle of wills.........because in the end the child will fight it and you will give up. Parenting a rebellious child is not easy, and sometimes we get exasperated and simply wish it was over already. But we must keep in mind that the child is not working with the same adult mentality that you are. She simply knows that she is hurt, angry, and wants attention. You hand out the punishments so you are the bad guy in all this.
The key is to shape her behavior with a series of rewards and punishments and to include her in the process. She is old enough to sit down and help you come up with the plan. The key is making it enticing enough that she wants to change.
The first step is to find some alone time.........maybe speak with her today and ask her if once the baby is in bed and asleep if you both can sit down and talk about some of the issues and how you can work together to solve the issues.
Once the baby is in bed you can explain to her that the recent upset is causing everyone stress and causing her pain that you do not want to see her in. Let her know that today is the day for change. You will likely see some attitude or her roll her eyes....let it go for now. Ask her a few key questions
This is a simple straight forward question. Ask her if she would like some new chances for freedom such as inviting friends over or going to places kids her age like such as movies with her friends once a month or rollerskating. Maybe it is the elusive cell phone or more time on the computer. What ever it is before you say no.........write it down.
Let her know that the ways things are going simply can not continue and from this point on you both are going to work as a team to change the home life. Let her know that there are going to be some new rules. For everyone including yourself. Grab a sheet of construction paper and write on it "Home rules" Then under that in smaller letters write your daughters name then your sons name then your name. Each person should have at least 5 rules, but no more then 10.
Ask her what she thinks some of these rules should be. On a piece of paper jot down her ideas as well as your own for each member of the family. Now is the time for negotiations. If she thinks the rule that she must clean her room everyday is to much....consider having her clean it once a week. The hitting should definitely be in there.....she can not hit and this is a important rule. Though this rule should also be on your sons list. While he is to little to understand it. You can place him in time out for one minute each year of age to show him hitting is not OK if he does this.
Explain to her that you are going to work on a rewards and punishment plan. Starting today she has all rights and privileges. She can play on the computer as long as you originally allow and may have back her original privileges. Keep in mind often it is easier to work on changing if you see what you have to lose. If she bucks up and starts following the rules then you should reward her. Being a good kid is not as easy as it looks mom.........sometimes it is so much easier to be bad Reward her once a month for effort. That does not mean that little slip ups should prevent a reward....it simply means that if the slip up happens that she loses the privileges for that day. If it is occurring more often then the privileges are gone for the week. If after six months she has shown a marked improvement consider getting her one of the prepaid cell phones. Often these are less then 15.00 and should she start acting up you have no contract to cancel.
Now for the hard part. Should she simply blow off the idea or continue on or start back up then it is time for some drastic intervention. As a parent we are required to do a few things by law. We must provide a roof over the child's head, a area to sleep and bath and food. We are not required to make that stay comfortable or fun.
I sometimes advise in serious situations a total strip down. This means a bed, linen, and the necessary bathing things. This means remove all toys, TVs, games, books (Except educational) etc from her. Inform her that she has no right to these things and these things are given to her to make her comfortable and happy but if she insists on being unhappy and making everyone around her unhappy then she has no right to them.
With this being summer this is far easier then while school is in session. Explain to her that once she begins following the rules consistently you will begin allowing her to have her things and rights back. For every week that she has shown marked improvement (And again allow for small slip ups) she may choose one thing she wants back. Should she have a bad week you may choose the item you are taking back. If she has 3 consistent back to back weeks then all her things are removed again.
Once she has earned back her things you can start with the rewards as outlined above. This may take a few weeks up to a few months to get her back in order. If you pay out for expensive name brands or allow her to pick her cloths inform her that should she not have her privileges back you will be making the choices in her clothes and there will be no name brand or expensive purchases. If she does not pick out her clothes this may be a great reward as well.
This is not going to be easy......that is a fact. But if you stick with your guns and stick with the program she will eventually tire of the games and get on track. She should have daily chores and this should be part of the rules as well. Consider offering a allowance as well for the chores as this should be a reward as well. If she does not do the chore then no allowance as well as this counts against her for the week.
I have seen parents who have been at their wits end.......after a few months on this program most of the children got back in shape real quick. Just remember that there must be a reward......if all she gets is a nod or a smile then why bother?