He is not sure whether he breastfed from her grandmother – may he marry her?
Question

                I have a sister of ‘breast feeding’. She was breastfed by my mother for sure, but I am not sure if I was breastfed by her mother. Nor I know how many times she was fed. The question is about her elder sister who was not breastfed by my mother for sure. She has a daughter and we have agreed to get married. Is it permissible to marry this girl whether or not it was proven that I was breastfed with her younger aunt?.
Praise be to Allah.Firstly: 
The mahram relationship 
    through breastfeeding cannot be proven unless there were five definite 
    breastfeedings, because of the report narrated by Muslim (1452) from 
    ‘Aa’ishah (may Allaah be pleased with her) who said: Among the things that 
    were revealed of the Qur’aan was that ten definite breastfeedings make a 
    person a mahram, then that was abrogated and replaced with five definite 
    breastfeedings. 
Secondly: 
If there is doubt as to 
    whether breastfeeding took place, or about the number of breastfeedings, 
    then the mahram relationship cannot be proven. 
Ibn Qudaamah (may Allaah 
    have mercy on him) said in al-Mughni (8/138): If there is doubt as to 
    whether breastfeeding took place, or about the number of breastfeedings 
    which make one a mahram, whether it was complete or not, then the mahram 
    relationship cannot be proven, because the basic principle is that it does 
    not exist, and what is certain cannot be altered because of doubt. End 
    quote. 
Based on this, so long as 
    you are not certain about the number of breastfeedings from the grandmother 
    of the girl whom you want to marry, then the mahram relationship cannot be 
    proven in the event of doubt, and the basic principle is that this girl is 
    permissible for you. 
But if it is proven that 
    you were breastfed (by the grandmother), then (the girl’s mother) is your 
    sister, and you are a maternal uncle of this girl through breastfeeding, so 
    she is not permissible for you. 
For the sister who was 
    breastfed by your mother, if she was breastfed five times, then she is your 
    sister and the sister of all your brothers, and her daughters are mahrams 
    for you, because you are their maternal uncle through breastfeeding. 
And Allaah knows best.