If it means anything, Yuina's mother was a futakuchi-onna, whose mother was a futakuchi-onna, whose mother was a futakuchi-onna... Etc. etc.  I'm assuming it was just the first futakuchi-onna who were transformed humans, really. XD

...I think the thing we'd figured out was that all the female offspring of a futakuchi-onna were more futakuchi-onna, whilst the male offspring were regular humans.  Or... Something. ^^;