Is nikah permissible with a foster sister or brother?

I have a question regarding nekah (marriage) between a foster sister or brother. Specifically, my mother breastfed the son of her sister 23 years ago, as I have gone through some of the answers about the same issue I got my answer to some extent based on ayats from Holy Quran and Hadith. But the part of question remained unanswered, which is my mother has been saying that I took Farhad (the son of her sister) for the purpose to breastfeed but I am not sure, about whether he drank my milk or not, since it was a long time ago. Can you please state is it permissible to make nekah with this person or not?