Twins AND half-sisters.

While planning and brainstorming for MEOW SISTERS, there are details I'm trying to work out (always!) that are giving me pause. 

I've pretty much decided this will be a story about sisters with the same mother, but different fathers.  They were raised separately, but are well aware of the other's existence. And, in fact, they spent a fair amount of time together when they were younger.

Recently, I've been frustrated with some of the backstory and the ages of the girls.  I know that there is a certain amount of leeway here, and that if I can present the motivations either clearly or vaguely enough, it won't be a problem for readers.  And yet, it bothers the hell out of me not to know the answers.

One idea I came up with is having the girls be twins.  Twins AND half-sisters!  When I was taking science classes from elementary school on, we weren't taught about this type of situation.  Twins were either identical or they were fraternal.  And in extremely rare cases, twins might be cojoined.   End of.   I'm certain this wasn't considered fact by science as a whole, since there was research being done in recent decades.  But the textbooks and teachers I had weren't discussing these things.  In fact, I remember being really confused about how it could be possible for a cat to give birth to a litter of kittens all having different fathers sometimes, but that the same couldn't be true for humans.  If it's possible to have fraternal twins (and it is, of course!) it always seemed to me that half-sibling twins could result.  You know, if the situation were such to make it possible.

A couple of years ago, I watched an episode of GREY'S ANATOMY where a woman had two uteruses and was carrying fetuses of different fathers.  That prompted me to read more about twins and discover that there are more variations on twins than I had ever known.  The one that intrigues me most -- and that might work really well in the context of my story -- is heteropaternal superfecundation.     This is where one egg is fertilized by the sperm of one male, and another egg is fertilized by the sperm of another male.  The fetuses are the result of one pregnancy, thus giving them the classification of "twins."  But, genetically, they are, in fact, half-siblings.   There are a few different ways this can happen, and they don't seem require a woman to have two uterusus either!

I don't know.  Maybe this idea will make writing this story more difficult than it needs to be.  Maybe I'll find that it won't work after all.  But I am intrigued at the moment.  I'm trying to decide whether this will strengthen the story I'm already putting together.  One thing I really need to figure out is how much knowlege doctors and labs had of this twin stuff during the early-90s when my characters would have been born.  Paternity tests are so common these days that you can buy them at Rite-Aid, mail in your cheek swabs to a lab, and get results quickly.  But what would it have been like sixteen years ago? 

Has anyone seen the concept of heteropaternal superfecundation come up in YA fiction?  Or in any modern fiction other than daytime television?  I'm very curious!