There's a book called Siblings Without Rivalry. I have NO CLUE how it is but maybe someone here has read it? Or you could look at reviews? My two bicker all the time. I've been wondering too if there's something I could do to help.
SAHM to Coralie, 1/09, Siri 9/10, Ellie Sue 9/12, Baby #4 due May 2014.