bridge sibling 

Your half sibling's, half sibling. When a child's parents seperate and go on to have other children, the relationship between those subsequent siblings. The original child acts as a 'bridge' between the two families.

This is Derek, my half-sister Mary's half brother. Mary and I share the same mother while Mary and Derek share the same father, he is my bridge sibling.

bridge sibling by GoJump April 25, 2016