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.
Example:
"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!"