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.