Is it obligatory to veil oneself from one's sister's son who is 12 years old?

A woman must observe hijab from a boy if he has reached twelve years of age, even if he has not shown other signs of puberty, because he is at the age of adolescence. An adolescent is permitted to enter upon non-mahram women without seeking permission, except during the three times when they lay aside their garments. However, the woman must observe hijab from him. His guardian is obligated to prevent him from looking at 'awrat (private parts). As for the adult, his ruling is the same as that of men.