The step sister of your step brother or step sister.
KAREN: This is my side-step-sister, Sherry.
KAREN: Yes, Sherry is my step sister's step sister.
SHERRY: Hi, Jane. Did you meet Alice?
SHERRY: Well, Karen & I are not genesisters at all. But, Alice is the step sister of BOTH Karen AND I. That's how Karen and I met -- through Alice.
JANE: Okay. I get it. Neat!