SIBLING LOVE

Every couple has little spats, but I have never seen these two really mad at each other.  I don't understand their relationship, but I have never seen two people get along as well. 
Addendum: Duke and Duchess are brother and sister.  At the 7 to 1 ratio, this year we three are the same age.  Next year they will be seven years older than me.  I don't display this sort of behavior that much any more and neither do they.  Duke visits every dog on the beach, but refuses to fight no matter what.  Duchess visits every small child.  She will fight if she is jealous, but only to the death.