Suspicion that engaged cousins are siblings from breastfeeding
16-1-2017 | IslamWeb
Question:
I heard in my childhood that my mother breastfed my cousin (father's nephew). However, as we grew up, this rumor faded, and I have been engaged to that cousin and am likely to marry him in a few months. Now, I have seen thrice in a dream that I am marrying my brother. It put me in severe doubts and revived the rumor related to the breastfeeding. I asked my mother and aunt (cousin's mother), they say that they do not remember. If it happened, it happened once or twice. Please guide me in this regard.
Answer:
All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad 
is His slave and Messenger.
If there is any doubt about this breastfeeding, then this does not prove the prohibition, which means that it is not forbidden for your cousin to marry you.
Ibn Qudaamah 
said, “If there is any doubt about the existence of breastfeeding or regarding the number of breastfeedings that make one prohibited and whether this number has been completed or not, then this does not confirm the prohibition, because the principle is that there was no breastfeeding, and certainty cannot be overruled by doubt."
For more benefit, please refer to fatwa 89967.
However, it is better to avoid marriage when there is a doubt about breastfeeding; the Fiqh Encyclopedia reads:
“Al-Khattaabi said in relation to breastfeeding:
“It is one of the impediments whose existence prevents the presence of the ruling at the beginning and at the end. It prevents the beginning of the marriage and stops its continuation – if it became known after the marriage – and if there is doubt about its occurrence, then this does not affect the marriage based on the rule: doubt is eliminated. However, it may be said that it is better to avoid it. The scholars stated that a person should not marry except a woman that is certainly lawful to him.”
It is essential that extreme caution is given to the matter of breastfeeding and its counts as this may lead to many evils, as a man may marry one of his female Mahrams (female relatives who are permanently prohibited for him for marriage) while he does not know it.
Allah knows best.