The Kagamines
The Kagamines are the only vocaloid group to have one of them mirror the other. Rin was made first and Len was made to look like her. The same person was used for their voices, a woman with a talent for changing her vocal tone. The Kagamines have always been sold together as a pair, but they are not twins, nor are they married. They did this to make sure they could have their own individual songs. As well have parts that were both sibling like and marriage like.