Praise be to Allaah.
If a woman breastfeeds a child under the age of 2 for five
feedings, then the rulings on relationships through breastfeeding are
established. She becomes his mother through breastfeeding, her husband
becomes his father through breastfeeding, and all her children and the
children of her husband, whether they are their own children or children
through breastfeeding, become siblings of this child who was breastfed.
In order to prove that this relationship of mahram exists, it
is not essential that the woman or the child should have intended it to be
so, or that she should know that he was not her child. With regard to the
rulings on breastfeeding, the scholars have mentioned issues from which this
ruling may be understood.
They said that if an infant girl crawled to a sleeping adult
and breastfed from her, then motherhood through breastfeeding is established
and the rulings on breastfeeding become applicable.
See: al-Mughni, 11/333; Tuhfat al-Muhtaaj,
Based on that, your father's wife is a mother to this boy
through breastfeeding and your father is a father to him, and you are a
sister to him.
And Allah knows best.