Is a cousin considered a sister by fosterage if the mother does not recall having nursed her, but the aunt remembers it, and there are situations recounted by their grandmother that indicate the fosterage took place?

The suckling that establishes prohibitation (of marriage) must consist of five full sucklings, according to the preponderant opinion. What you mentioned about your cousin suckling from your mother twice is not enough to establish mahramiyyah (prohibition of marriage), even if these two sucklings were confirmed. Furthermore, the testimony of the maternal aunt and grandmother does not establish suckling according to the Shafi'i, Hanafi, and Maliki schools of thought if the suckling was not widely known among people. Therefore, it is permissible for you to marry her if no other reason for prohibition is established between you.