I have the urge to tell my introverted sister that my nephew is under socialize.
I would you want to do that? Is your nephew want to socialize more?
I don't know
Socialization can be important for a child, but there may be issues with your nephew that you don't know about. Don't carelessly mention your nephew socializing without knowing if there's any issues. You could hurt your sister's feelings and cause a rift in your family.
My family always picks the wrong side. I should transfer to another family.
I don't think transferring to another family is going to solve your issues. I think you simply just need to have some communication with your sister see what is actually going on with your nephew.