The preposition "en" can also denote participation in abstract and collective concepts.
For example:
"soy ignorant en todo" (I am ignorant of everything), "valiente en la pelea" (brave in a fight), "hábil en engañar" (skilled in deception).
From this sense of participation in a larger abstract concept comes a set of related verbs that express thought and belief that function with the preposition:"creer en" (to believe in), "esperar en" (to wait/hope in), "confiar en" (to trust in), "dudar en" (to doubt in).
Note that the phrase "creer a mi hermano" means "to grant him credit in that which he says," while the phrase "creer en mi hermano" denotes a much more extensive belief in my brother which includes not only what he says, but also belief in his talents, his honesty, and all his other qualities. The preposition "en" functions the same way for the other verbs of thought that are have been listed.