antipodal sibling
A step-sibling of a step-sibling.
Example:
"So how are you Andrew related to each other?"
"My dad married Mary's mom and Andrew's mom married Mary's dad after Mary's parents got divorced. So mary is my step-sibling and also Andrew's step-sibling. That makes me and Andrew Antipodal Siblings."
"So how are you Andrew related to each other?"
"My dad married Mary's mom and Andrew's mom married Mary's dad after Mary's parents got divorced. So mary is my step-sibling and also Andrew's step-sibling. That makes me and Andrew Antipodal Siblings."