Like Two Peas In A Pod

( they fit together very well; they are comfortable with each other ... )

Two things that are similar and go together well are like two peas in a pod. Example: "He and his sister are like two peas in a pod."

Peas fit together comfortably in a pod where they grow, and two people who get along well together are like two peas in a pod. Example: "Your brother and his wife are so happy and comfortable together." Reply: "They are like two peas in a pod."