The mahram relationship is not established if there is any doubt about the number of breastfeedings
Question

                My stepmother breastfed the son of my full brother but she is not sure about the number of times she breastfed him, and she is not sure whether this breastfeeding establishes the mahram relationship or not, because this nephew wants to propose to my daughter.
Please advise us, may Allah reward you with good.
Praise be to Allah.As there is some doubt about the matter, and you do not know whether it was five breastfeedings or four, or less than that, then the basic principle is that the mahram relationship does not exist. As that is the case, there is nothing wrong with this marriage, and this breastfeeding concerning which there is some doubt is not considered to establish the mahram relationship.
And Allah knows best.