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