A few years ago, my aunt (my father’s wife) breastfed her neighbour’s son from both sides until he was satisfied and slept from night until morning, which is a sign that he was full. My question is: When my aunt breastfed the child she thought that it was her son, because they were the same age (4 months) and the same size, i.e., it was breastfeeding by mistake, but breastfeeding took place on more than 10 occasions when the child was fully breastfed (i.e. was full) and there were witnesses. Does the fact that it was a mistake count for anything?
I hope that you can clarify because there are strong ties between us.
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, 3/492.
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.