A step-
sibling of a step-
sibling.
Here's an example :
A & B had Child 1
A & B then got divorced, A married C and B married D.
C had Child 2 and D had Child 3 from their previous
marriage.
Couple A&C and couple B&D now share childcare responsibilities of Child 1, 2 & 3 and often babysit them together.
So, while none of the three children shares blood, Child 2 & 3 have a common Step-sibling, Child 1.
Child 2 & Child 3 could be called each other's Antipodal Siblings.
"How are you and
Jimmy related?"
"Well, we're Antipodal Siblings. Him and I share the same step-sibling, Lucy. Lucy is my step-sister cause her mom married my
dad and Jimmy's step-sister cause her
dad married his mom. We're one big blended family!"