See on Mar 3:18. Compare Mar 13:3; Joh 6:8; Joh 12:22.
Simon Peter's brother
The mention of Simon Peter before he has appeared in the narrative indicates the importance which the Evangelist attaches to him. It seems to assume a knowledge of the evangelic narrative on the part of the readers. See a similar instance of anticipating what is subsequently explained, in the mention of Mary, Joh 11:2.