Is it permissible for me to marry my fiancé, my paternal cousin, knowing that his elder brother was nursed with my elder brother, and my brother who is older than me was nursed with my fiancé, and I had seen dreams before I knew about the nursing situation indicating that he had become my brother?

There is no impediment to your marriage with your paternal cousin, as long as there is no certainty of prohibitive suckling between you, which consists of five full sucklings for each of you. As for Istikharah, Allah will choose what is best for the one who performs it, and a dream does not entail a legal ruling.