cross-sibling

plural cross-siblings

1. anthropologyAn opposite-sex sibling or parallel cousin.

   Ifaluk society even forbids smoking in the presence of one's cross-siblings, because it involves movements and gestures with sexual connotations.

   1998, Charles William Nuckolls, Culture: A Problem That Cannot Be Solved, page 65

2. neologismThe half-sibling of one's half-sibling; a person with whom one shares a half-sibling, but no parents.

   As cross-siblings we're not in any way bound to each other; our relationship is optional, we don't have to get on – we just do.

   2013 December 28, Janelle Butterfield, “I bonded with my cross-sister – we share half-brothers”, in The Guardian, archived from the original on 29 Dec 2013