Are Rin and Len twins or not?

"The development of the Kagamines began when Crypton formulated the concept of making a pair of female and male voicebanks. The first idea was for two voices of a girl and her mirror image of the opposite gender, just like twins.

Crypton had the intention to distribute them as twins, but this was not adopted. After seeing many works that depict their relationships differently, Crypton announced in a magazine interview that those would meet their ideal. However, Crypton's final announcement was that they were neither siblings nor lovers. During Append production, Wataru Sasaki (aka. Wat) described them as having one soul between the two of them. Wat also described that the Append boxart makes the pair seem like twins,due to the fetal position they were in."

This is from the wiki itself so as it already said,they are not twins. Nor are they siblings in actuality they're the same person. Just Rin copied over as a male basically. They can be twins or siblings in the songs or lovers in other songs. But they're not related to one another.

I'd like to add that 'kagami' means something like 'mirror' and 'kagamine' should mean something along the lines of 'mirror sound.' That could mean a bunch of things so take it how you want, but nothing about the personality or background info about vocaloids are canon, or official. We can do whatever we want with them and no one *should* throw a fit.

It really helps to look at their names if it's a traditional one. They tell you more than you think.

Well, they're shown as twins throughout the fan base, but really, they're supposed to be mirror images. Crypton has shown them as twins many times, so you could believe both~ Kagami: Mirror. Ne: Sound. Rin: Right. Len: Left. Mirror sound right, Mirror sound left.