in a sense, neither. kei said in some interview or something that he designed them thinking "mirror images" to some extent, and that is somewhat canon, but the canon we have for miku is "a songstress in a future with no music," and we have people like mothy who seem to think of her as being from the past, even if that is for his stories. some people also disregard lapis being a fairy.
tl;dr: there is canon information, but canon is often ignored in the vocaloid fandom, and for good reason. it brings more flexibility. they're whatever you want them to be.
For me it would have to be brother and sister, even though they are just mirror images. Either way, in my opinion they look to much alike to be" that way". It just feels odd.
