A man marrying a girl who was nursed with his sister
We have reviewed request no. 742 for the year 2005 which includes the following question:
My sister has a sister-through-nursing, so is it permissible for me to marry this sister through-nursing?
Thank you for your question. This is an important issue, which is not always understood by many people. This issue essentially relates, first and foremost, to the woman who breastfeeds a child. When a woman breastfeeds a child it becomes her child-through-nursing (breastfeeding), and thus she becomes its mother. This is on the condition that it occurred before the child was two years of age, and she breastfed at least five full feedings. Once this relationship is established, the child becomes the sibling of the woman's children, both her biological children and those through breastfeeding (with the above-mentioned conditions). Because those children are now considered brothers and sisters, they are not allowed to married each other.
Outside of this situation, a person is not considered the brother or sister of those children (again, either biological or through breastfeeding), thus they may marry them.
So to apply this to your situation: Unless your biological mother was the one who breastfed this woman, or you and her were both breastfed by the same woman, than she is not your sister-through-nursing, thus you may marry her.
God the Almighty knows best.