My mother was nursed twice by a woman, and I want to marry that woman's daughter. What is the ruling on this?

Scholars have differed in determining the amount of suckling that establishes prohibition (mahramiyyah); some did not differentiate between a little and a lot. Consequently, the daughter of the woman your mother breastfed would be your maternal aunt through suckling and thus unmarriageable.

The most preponderant opinion is that less than five sucklings do not establish prohibition. Therefore, there is no impediment to marrying the aforementioned girl. However, refraining from marrying her is more cautious for one's faith and indicative of piety, based on the Prophet's (peace and blessings be upon him) saying: "Leave what makes you doubt for what does not make you doubt."