A word for someone who you share a half-sibling with

This is probably best explained with an example.

Alice and Bob have a child, Erin. Bob and Carol have a child, Frank. Carol and Dan have a child, Grace. Clearly, Erin and Frank are half-siblings, because of their father, Bob. Similarly, Grace and Frank share their mother, Carol.

Erin and Grace have no parents in common, and are thus not half-siblings. However, is there a term for their relation?

Solution 1:

I just read of a term called cross siblings. Where you are not legally or biologically related but you share a half sibling. My kids are tech half siblings but we never call them that because they are growing up together with me. But they both have half siblings, and the half siblings have half siblings. And it can make your brain hurt trying to explain it all lol. But because of this I've googled it a few times. And came across a girl in the UK who uses the term cross sister when it comes to the girl she shares half brothers with, I then googled it and have found it has multiple hits. I don't know how legit the term is but sure is easier to say than brothers half sister or half brothers half sister.lol

Solution 2:

The argument by @ermanen is not correct.

A half-sibling is a "half" because they have half of their parents in common.

In the example provided, Erin and Grace have no parents in common. They are not related in a manner that would be designated half, quarter, or whatever.

The only relationship they could have is through marriage, meaning they are stepfamily. Let's see if there is a relationship. Yes there is.

Erin's father Bob remarried to Carol. So Carol is Erin's stepmother. A stepparent's child through another marriage is a stepsibling.

So what we have is Grace and Erin are stepsisters of each other.

And Frank is a half-brother of both Erin and Grace, having one parent in common with each of them.

A "stepsibling" is the word for someone you share a half-sibling with.

Solution 3:

If we use sibling-based terminology, we can come up with quarter sibling (as we have full sibling, half-sibling and three-quarter sibling).

(Note: It would be a colloquial (or even jocular) coinage rather than biological. Identical twins share 100% of their DNA, full siblings share 50% on average, three-quarter siblings share 37.5% on average and half-siblings share 25% on average. I think we can only go below 25% by leaving the realm of siblinghood and stepping into the realm of cousinhood; as first cousins share around 12,5% of their DNA. Thus, quarter sibling seems not to be applicable to biology.)

Urbandictionary defines quarter brother:

my half-brother's half-brother

Jason and Lydia's son is Sam. Clai and Lydia's son is Will. Jason and Maureen's son is Davis. Will and Davis are quarter brothers.

and quarter sister:

The half sister of my half sister or half brother.

Amy and Randy had Josie. Randy and Patty had Vanna. Patty and Bob had Andy.

Vanna is Josie's half sister.
Vanna is Andy's half sister.
Josie is Andy's quarter sister.
Andy is Josie's quarter brother.

However, the following excerpt got me thinking...

But if A is half sibling to B and B is half sibling to C, A can have virtually any relationship to C, with quarter sibling being among the least likely. Social relationships may not be based on such objectively measurable unidimensional scales as weight and size, nor even such indirectly measurable attributes as strength or "confidence."

Behavior, conservation, and ecology, Volume 2 by G. Mitchell, Joseph Erwin, Daris Ray Swindler

Bonus:

nonsibling: (biology) Any individual that is not a sibling [Wiktionary]