At this age, the relationship of your child with his sibling/cousins or close neighbors would last for long as compared to other people as they grow together. The positive benefits of establishing warm and positive relationships may last a lifetime.
What you need to know:
At this point, these interactions of your kid are emotionally charged relationships defined by strong, uninhibited emotions of a positive, negative and sometimes ambivalent quality. It is a safe and secure place to learn how to interact with others who are interesting and engaging playmates, to learn how to manage disagreements, and to learn how to regulate both positive and negative emotions in socially acceptable ways.
These are ways to develop an understanding of social relations with siblings, cousins or neighbours who may be close and loving at times and nasty and aggressive at other times. Further, your kid has many opportunities for him to use his cognitive skills to convince others of his point of view, teach or imitate the actions of his cousins/siblings/neighbors.