I think its " normal " my children are not that old i just wanted to wish you the best of luck.

Maybe try more family activity where they have to get on ?

I have 2 boys 14 and 16 who were always fighting and picking at one another - then one day they had a 'real' fight. I let them get on with it - end of fights the little one (who usually started it) lost and hasn't picked again

Never takes sides, but do listen and offer understanding and interest. Some great books to help with siblings rivalry (which is normal anyway) are: Siblings Without Rivalry and Raising Our Children, Raising Ourselves.