Probably brother and sister.
Here's Rika at home with her husband Charles and father Christian: "United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MSGP-B23 : accessed 1 June 2018), Ricka Alwardt in household of Charly Alwardt, Election Districts 1 Kingston city Ward 2, Ulster, New York, United States; citing enumeration district (ED) 98, sheet 3A, family 59, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,241,169.
Here's John, son of Christian, marrying Sophia Lindhorst: "New York Marriages, 1686-1980," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:F6S8-LG7 : 10 February 2018), Chris. Pufpaff in entry for John Pufpaff and Sophia L. Lindhorst, 09 Aug 1911; citing reference 418; FHL microfilm 842,643.
John's profile might have some spouse confusion?
Here's the Pufpaff family, Christian and Maria, along with Fred'a, 5, and John, 3, arriving in New York in 1863: "New York Passenger Lists, 1820-1891," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:QV33-FBXQ : 12 March 2018), Chr Puffaf, 1863; citing NARA microfilm publication M237 (Washington, D.C.: National Archives and Records Administration, n.d.); FHL microfilm.