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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?

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    Not sure if there's a specific term for that relationship, but they'll probably be referred to just as "extended family".
    – Nicole
    Commented May 5, 2015 at 2:30
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    You didn't specify which of the parents (if any) are currently married, and to whom. This matters because if there were stepfather/stepmother relationships, one could use these to construct a description. Otherwise, Erin could only be described as Grace's half-brother's half-sister. And Grace is Erin's half-brother's half-sister. These would of course be greeted with great puzzlement by any listener.
    – Brian Hitchcock
    Commented May 5, 2015 at 6:28
  • +1  just for using intuitive names (compared, for example, to Jacob, Mary, Joshua, Samuel, Morgan and Ava).
    – Scott - Слава Україні
    Commented Feb 18, 2018 at 20:11

3 Answers 3

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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

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  • Hello ABurns, welcome to the site! Thanks for contributing with this answer; it seems to be the best-fitting term for the situation described. I think you could improve it further by adding a link to some source using this term; I found this Guardian article using it for this exact type of situation: theguardian.com/lifeandstyle/2013/dec/28/…
    – herisson
    Commented Jun 3, 2015 at 2:19
  • In the Guardian article, those sisters say that they just coined it. I couldn't find any other supporting source. It actually has another meaning in biological contexts. An example article: ncbi.nlm.nih.gov/pmc/articles/PMC3290407
    – ermanen
    Commented Jun 3, 2015 at 2:23
  • @ermanen: I did find a few other sources (a Familypedia article and a Yahoo question), though of questionable quality and possibly derived from the Guardian article. In the article you link, it seems to just be being used compositionally to basically mean "between siblings". The term seems to have a third meaning in sociology, where it may refer to siblings of opposite gender as far as I can tell...
    – herisson
    Commented Jun 3, 2015 at 2:59
  • Here's a link for the sociological use: books.google.com/…
    – herisson
    Commented Jun 3, 2015 at 2:59
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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.

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  • Your argument about my argument is incorrect. Please read carefully. Also, stepsiblings don't necessarily share a half-sibling.
    – ermanen
    Commented May 7, 2015 at 1:56
  • @ermanen - We can let the readers/voters logic it through and decide whose argument is correct. You are right in saying that stepsiblings don't necessarily share a half-sibling. But, if you have two people who do share a half-sibling, such as the original poster asked, then they will be stepsiblings.
    – lkessler
    Commented May 7, 2015 at 3:28
  • +1 for being the first to post an answer bringing up this possibility. However, the original post mentioned nothing about marriage, so the assumption that Bob remarried Carol may or may not be true strictly speaking.
    – herisson
    Commented Jun 3, 2015 at 2:16
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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]

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  • Following the same terminology doesn't really make sense, though.
    – herisson
    Commented May 5, 2015 at 3:00
  • Also note that the given definitions also would include any full siblings you may have. As well as yourself.
    – Kevin Mills
    Commented May 5, 2015 at 3:05
  • Vanna is Josie's half sister. Josie is Vanna's half sister. Vanna is Vanna's quarter sister. The same argument holds for full siblings. Of course, this can be fixed simply by adding a qualifying statement to the definitions.
    – Kevin Mills
    Commented May 5, 2015 at 3:24
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    "Quarter sibling" would probably be used by ecologists/biologists for a relationship with 17.5% relatedness on average. That's why they say that if A and B are half-siblings, and B and C are half-siblings, it's unlikely that A and C are actually quarter-siblings.
    – herisson
    Commented May 5, 2015 at 3:27
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    @sumelic: I think the coinage might be just following the naming instead of the logic behind it (so not about the percentage of gene share). It seems like just a colloquial coinage. I'm not sure how it can be applied to biology. I couldn't find any source that uses this word in a biological context. (except the ecological context I found.) Beside biology, "quarter brother" and "quarter sister" were coined to define what OP is asking.
    – ermanen
    Commented May 5, 2015 at 3:32

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