What is the correct course of action regarding a woman who claims to be a sister through رضاعة (breastfeeding), based on the statement of her deceased mother, while the mother does not recall the incident and rules out its occurrence?

The prohibitive degree of fosterage is established by the testimony of a trustworthy woman who breastfed five times during the first two years of life, based on the hadith of Uqbah ibn al-Harith. If the nursing woman is trustworthy and a reliable man or woman testifies to her having breastfed five times, then fosterage is established.

If the woman who claims to be your sister informs you that her trustworthy mother breastfed you five times, then the prohibitive degree of relationship is established. However, if the number of feedings is unknown, the prohibitive degree is not established.

The prohibition depends on the integrity of the deceased woman (the foster mother) and the certainty of five feedings. In case of doubt, one builds upon certainty, and avoiding doubtful matters is preferable. The precautionary measure would be not to marry them or intermingle with them.