Sibling squabbles

How do you correct sibling squabbles and encourage them to bond and build friendships with each other? I have 4 kids boys 13, 12, and 9 and a 7 year old girl. It is mostly the boys that fight. It isn't anything too intense they just pester each other and then end up hurting other by digging their nails into each other or hitting. They are respectful towards my husband and I but not with each other. They love the Lord so we do point them to the word, but would love any other tips you have.