I heard from my brother that the final and most challenging test for a Mu’min and Muslim is resisting the temptation of the opposite gender and avoiding Zina. Is this true?
It tends to be a big challenge for people, especially in youth. Often, as people get older, that particular temptation becomes less, and other temptations come to the forefront. (Exceptions apply!)
However, although that is a common temptation, there is no one single challenge that everyone faces equally - our temptations are as diverse as ourselves. Possibly, your brother is going through a difficult time managing this aspect of himself, and this is why he said that.