Is breastfeeding that renders marriage unlawful established by the report of the foster mother’s daughter (the sister) on the authority of her deceased mother, in the absence of other witnesses, and the presence of other inconclusive reports from a second sister, and the denial or lack of recollection from family and relatives?

The preponderant opinion among scholars is that the testimony of a single woman regarding suckling is acceptable if she mentions the details of the suckling, such as the time of suckling, the number of times suckled, and the milk reaching the stomach of the suckling infant. Accordingly, your sister's testimony is not sufficient to prove suckling due to the lack of details. However, if your sister is not suspect and is truthful, and you wish to complete the marriage, it is more prudent for you to abstain from this marriage as a precautionary measure.