See Kagamine page - Kagmine Rin \ Len page and the concept section.
As they were under development, Crypton had the concept of making a pair of male and female voicebanks. The first idea of the concept was twin voices of a girl and her mirror image of opposite gender, just like twins. They have no set relationship, they can be twins, lovers, friends, siblings who are lovers.
The Kagamine's are seen as "Mirror Images" of one another, neither relating to one another as a sibling in a human sense. However, this does not stop the fandom or producers from portraying them as such. A rumor says Crypton is currently selling them as "twins" to the American audience, unlike in Japan where their relationship has never been defined.
- See also