Is it permissible to marry the daughter of a maternal uncle if there was breastfeeding between them, and the family differs on the number of رضعات (suckles) (three, one, or an unknown number)? And is it permissible to marry her sister?

If one doubts the existence or number of sucklings, prohibition is not established. The suckling that causes prohibition, according to the preponderant opinion, consists of five sucklings within the first two years of life. Therefore, you are permitted to marry your maternal cousin if the situation is as you described, and you are permitted to marry her sister, but it is not permissible to marry both of them simultaneously. However, it is more cautious to avoid marrying someone who was suckled by your mother, as a way of resolving the difference of opinion.