The Ninth Circuit determined that step-children of U.S. citizens do not derive citizenship under the immigration laws, which provides citizenship only to the naturally born and legally adopted children of U.S. citizens. In so doing, the Ninth Circuit agreed with the Board of Immigration Appeals' decision in Matter of Guzman-Gomez.
The full text of Acevedo v. Lynch can be found here: http://cdn.ca9.uscourts.gov/datastore/opinions/2015/08/24/12-71237.pdf