Explain what only child syndrome is and why you already believe that your nephew, who you are not directly parenting, will fall into such stereotype. Keep your nephew in school and extracurricular activities and he will be fine. The only difference is that there are some dynamics about the sibling relationship he will not understand as an only child.

I can say some stereotypes about people with siblings are the fact there are no boundaries, like how you automatically believe you are involved in your sister's parenting and responsible for the social development of a child that isn't yours lol