cross-sibling

noun2

1. •
   An 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
2. •
   The 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