Is it permissible for me to marry Nisreen, knowing that her father, Hussam, was suckled by my mother, but my mother does not recall the number of sucklings?

According to the majority of scholars, the nursing that establishes prohibition (mahramiyyah) is five known sucklings. If there is doubt about the nursing or its number, prohibition is not established. As for the Malikis and those who agree with them, they do not stipulate a number of sucklings; they deem it sufficient for the woman's milk to reach the child's stomach within the two years of nursing. Therefore, it is advisable to refrain from marriage in such a case, based on the Prophet's (peace be upon him) command to a man to separate from his wife because a woman claimed she had nursed both of them.