Ephraim was the brother of Manasseh, and the son of Joseph and Asenath, Pharoah's daughter. Jacob adopted the two sons as part of the tribe of Simeon and Reuben. Ephraim received the blessing of the firstborn, although Manasseh was the eldest, because Jacob foresaw that Ephraim's ancestors would be greater than his brother's (Genesis 48:13-20).
Sources: Bridger, David. Ed. The New Jewish Encyclopedia, NY: Behrman House, 1976; Navigating the Bible II