At the time the child is born, interaction skills begin between the people that are close to them i.e. mother and father, grandparents, brothers and sisters. The child as young as they are, communicate back by interacting with them by smiling, holding a finger, reaching out to hold a piece of skin or  clothing, crying with different internation patterns for food, attention, or to indicate discomfort etc.

If these normal patterns of basic interaction are disrupted it can become very difficult to communicate. For example if the child is unable to respond to eye gaze appropriately then you're not sure whether they are responding to you or not. This is particularly difficult for young  brothers or sisters who rely on these cues even more than  adults. If these skills are still missing as the child gets older, reaching 6 to 12 months say, then it can in fact further disrupt communication development and these skills need to be taught to enhance communication.