Spend time one-to-one to let them feel loved and unique
Choose the right moment to spend quality time one-on-one with each of your children: an afternoon's shopping with your daughter, a football game with your son, a walk or a trip to the cinema. It doesn't really matter what you do as long as you spend quality time alone with your child. As long as you make it clear that everyone has their turn at doing things with mum, you'll find the atmosphere between siblings is far more relaxed.
Take care to give everyone their turn without leaving too much of a gap in between, otherwise the child who hasn't had their turn yet will feel as if their brothers and sisters are getting better treatment than they are. This will cause sibling rivalry which, of course, leads to fighting!