Their relationship is up to the user. Officially they are "twin" voicebanks, but Crypton has long since abandoned the idea of establishing a relationship between the two. Even the word "Twin" can loosely be interpreted as just "two".
Kagamine Rin and Len have no official relationship. They can be twins, lovers, mirror images, and any other relationship depending on the song.
Some people like to think of them as a couple. However, others like to think of them as twins, and some like to to think of them as mirror images, and many like to think of them as seperate, unrelated people entirely. Sometimes, people may even think a combination of these (i.e. the forbidden love between two siblings). It's honestly up to you.