Two children with close ties

January 12, 2006 7:27 PM

Two sisters give birth. The two children have the same father.

Obviously, the two children are very closely related. But they are not brother/sister related. Technically, it may be said they are half-brother/half-sister to each other. But that seems insufficient.

What are they to each other? Is there a word for this?

I don't know how this question came up... I was thinking about unusual filiation possibilities...

I imagine, though, that this type of kin combination may have been somewhat common in our distant human past, our hunter-gatherer past, when polygyny was common.

posted by amusem to Human Relations (20 answers total)

So...they are both half-siblings, as well as cousins, to each other. Odd, but surely not too bizarre back in the earlier days, as you noted. Doubt if there is a single word or phrase that would accurately describe their relationship, though.
posted by davidmsc at 7:35 PM on January 12, 2006

its half brothers! they have the same father! they are also first cousins becasuse their monthers are sisters.

you may be thinking of "incest", but techniclaly this is probably not incest.
posted by Izzmeister at 7:35 PM on January 12, 2006

They'd be cousins too.
posted by pompomtom at 7:36 PM on January 12, 2006

Not too bizarre back in the early days, and can still happen today... Jaidyn Leskie, a child murdered in country Victoria in a famous case a few years back, was half-brother and also cousin to another child as his dad had fathered kids on two sisters.
posted by andraste at 7:46 PM on January 12, 2006

gross.

they're half siblings through dad and cousins through mom. They're kind of like 3/4 siblings.

My mother is a twin, and my cousins & I like say that we're genetically half siblings since our moms are genetically the same person.
posted by mdn at 7:50 PM on January 12, 2006

In a number of polygamous tribal societies the preferred marriage pattern is for a man to marry two (or more) sisters, who are also his cross-cousins. A cross cousin is the child of your mother's brother or father's sister, while the children of your mother's sister or father's brother are parallel cousins. In such societies, the word for 'sibling' and 'parallel cousin' are generally the same, while the word for 'cross-cousin' is different (sometimes also means 'sweetheart'). See how it makes sense? If your father married your mother's sister too, then your parallel cousins are also your half-siblings!

So even in such societies, there is no special word for the relationship you describe, but the question of (parallel) cousin vs. sibling doesn't arise, as they don't have separate words.
posted by nomis at 7:53 PM on January 12, 2006

In the Torah, (Bible), Jacob married two sisters (Rachel and Laah), making the twelve tribes of Israel (some of them) brothers and children of sisters.
posted by Izzmeister at 8:03 PM on January 12, 2006

I think they're actually more than 3/4 siblings. Siblings have 2/2 shared parents and 4/4 shared grandparents (6/6 shared ancestors). Half siblings have 1/2 shared parents and 2/4 shared grandparents (3/6 shared ancestors). These kids would have 1/2 shared parents and 4/4 shared grandparents (5/6 shared ancestors). So I think they're 5/6 siblings.
posted by scottreynen at 8:06 PM on January 12, 2006

And if the mothers were half sisters, the kids would be 4/6 siblings. Cousins are 2/6 siblings, because they share 2 grandparents and no parents. And if the mothers were half sisters and the fathers were unrelated, the kids would be be 1/6 siblings (or half cousins).
posted by scottreynen at 8:13 PM on January 12, 2006

Siblings share half of their “unique” genes, 0.5= 0.25 (common father) + 0.25 (common mother) (ala kin selection theories). In this case they share 0.25+(0.5*0.25) = 0.25(1+0.5). How much “sibling” are they: (0.25(1.5)) /(0.25*2) = 1.5/2 = 3/4=75%. Hence I support the conclusion of mdn, scottreynen. I’ve been wrong before though, and this is not my field.
posted by JeNeSaisQuoi at 10:33 PM on January 12, 2006

A cross cousin is the child of your mother's brother or father's sister, while the children of your mother's sister or father's brother are parallel cousins.

My mother's brother is married to my father's sister and they have a few kids. So, um, what happens when you cross the streams?
posted by Pryde at 11:58 PM on January 12, 2006

I've seen on specials (and also late at night on shows like Montel) instances where identical twins married identical twins. How's that for a brain stumper!
posted by radioamy at 12:17 AM on January 13, 2006

Izzmeister, what are you saying? Now Israelites are brothers, too?
posted by ouke at 12:53 AM on January 13, 2006

nomis: huh, I didn't realize there was actually a term for my parents. (my dad's mum and mum's dad are siblings) Interesting.
posted by divabat at 3:14 AM on January 13, 2006

I've heard the children of two sets of siblings called double cousins.
posted by orange swan at 6:45 AM on January 13, 2006

Yeah double cousins, I've heard that too.
posted by JanetLand at 6:54 AM on January 13, 2006

There's a similar thing in my family. My maternal grandfather was killed in the war and his brother then married my grandmother. My mother always called the kids they had her sister and brother, except sometimes when she was pissed off at them. Then she'd call them her 3/4 sister and 3/4 brother. Clearly they're more than half siblings so this seems reasonable, but I'm not sure if there's a proper term for the situation.
posted by Decani at 7:30 AM on January 13, 2006

scottreynen they also share 8/8 great grandparents so they would be 13/14 siblings. Keep going back and the number approaches one. So I think you have to do the computation like jenesaisquoi did and 0.75 sibling is correct.
posted by jockc at 11:26 AM on January 13, 2006

You can't tell how much genetic info the two offspring share, incidentally.

You can make a guess about the most likely value, but the actual value depends on how much genetic info the two mothers share, and that could range from 100% (the mothers are identical twins) to a rather small value (the only genetic contribution the mothers share in common is their paternally-derived X chromosome.) (It could be zero, if one of the mothers had uniparental disomy X).

The most likely answer is that the mothers have 50% of their genetic material in common. In this case, each offspring are likely to have 25% of their maternally derived genetic material from the 'common' pool; again, the most likely result is that only half of that material is in common with their cousin-sib.

So the most likely amount of relatedness is 50% paternal contribution in common + 12.5% maternal contribution = 62.5% in common. (If they're both the same gender, it's a little higher because you know they share their paternal sex chromosome; if they're different genders, it's a little lower because you know they don't.)
posted by ikkyu2 at 2:41 PM on January 13, 2006

As far as unusual filiation possibilities go, what do you call fraternal twins who have different fathers?
posted by electro at 5:37 PM on January 14, 2006