If a Child is Weaned, His Being Breast-fed Has no Effect
Question :
A four year-old female child was breast-fed by a woman. who is a mother of a one year-old boy; will she be a sister to this boy who is three years younger than her?
Answer:
This breast-feeding has no effect. The majority of scholars hold the view that once a child has reached two years of age, there is no effect from his being breast-fed after that. Among them are those who hold the view that it is the weaning of the child which is taken into account. So if the child was weaned, even if it occurred before two years, his being breast-fed would have no effect; and if he was not weaned, even after two. years, then his being breast-fed would have an effect. But in most cases, a girl who has reached four years of age has been weaned, so her being breast-fed would have no effect.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Page 145