twins competing and fighting

Hi, I have read some of your replies to other parents about sibling rivalry. HOwever I wondered what you would say about twin rivalry. I have 5 year old twins. A boy and a girl. As well as a nearly 2 year old boy and a newborn. We have several issues. One being that the older 2 will not go to preschool, school or day care. They have been with me or my husband and will spend the occasional few hours with my sister or with a grandparent. They are incredibly demanding - they want attention nearly all the time. I have always tried to give them what they need, they were breastfed til 3.5. We co-slept until they wanted their own beds. We havent forced them to go to preschool although we have tried. My biggest problem has been since they started fighting with each other. They are so competitive that if one has something the other wants it too even if they dont really want it. For example, at dinner time one will say no to more potatoe until the other has some and then they will decide they do want some after all. They will fight over the same chair, the spot in the car, the swing, whatever the other one has. They figh over who is going to hold the baby to the point where i have had to remove the baby for its safety until they calm down. When they were little I tried to have 2 of most things so they didnt have to share everything. We figured they shared so much as it was. I hope doing this hasnt created an expectation of always having what the other has. At the moment they seem bored and with the toddler and the baby I am really unable to give them the stimulation they want. I think they should go to school of preschool but emotionally they arent ready. We are contemplating homeschooling but the need for attention is so high my husband and I are burning out. The fights go on all day (as well as lots of playing happily) and i just dont have the skills to help them sort them out. They get very physical with each other and have done for a few years now. I know they get angry and just cant stop themselves from hitting out but I dont respond well. I just get angry. And even though I know this is reated to my relationship with my older brother knowing that does not change my response. I find myself yelling before i even know what is happening. I try to be calm but when they keep hurting each other i get very upset. I also feel totally inadequate to give them what they need. I feel like i have done everything i can and they are still needing more. HOw can I get myself back to a place where i have something to give them. I know they need me, my attention, but i am not able to fulfill their demands especially not at 6am when the baby needs feeding and the toddler needs dressing. Now i am raving, but there are a few issues here and i am trying to cover the main points. Thanks