Is it permissible for me to marry the daughter of the elder sister of my foster sister, regardless of whether my fosterage from her mother is proven or not?

The prohibition by fosterage is established with five known sucklings, based on the hadith of Aishah (may Allah be pleased with her) that among what was revealed in the Quran was: "Ten known sucklings prohibit," then they were abrogated by "five known sucklings." If there is doubt about the existence of fosterage or the number of sucklings, the prohibition is not established, because the default is its absence. Accordingly, as long as you doubt having been suckled by the grandmother of the girl you wish to marry, the prohibition is not established by doubt, and the default is that this girl is lawful for you. However, if your fosterage were proven, then her mother would become your sister, and you would become a maternal uncle to this girl through fosterage, so she would not be lawful for you. As for the sister who was suckled by your mother, if her suckling amounted to five sucklings, then she is a sister to you and all your siblings, and her daughters are unlawful for you because you are their maternal uncle through fosterage.