are Rin and Len brother and sister or what

No, they are considered "Mirror Images", hence the "Kagami" in their last name with means "Mirror". I guess the publishers of Vocaloid changed them to "Mirror images" with all the Twincest out there. xD

They`re Mirror images of themselves , but you can say they`re twins . cause twin = 2 . and yeah . Fans think of them as twins.

KEI stated in an interview that he specifically did not design them as twins, and was told the two are more like mirror images of eachother and design them as such by CFM. He said since he didnt design them at the same time (Rin came first), Len ended up looking a lot like Rin and was mistaken as her twin brother. He did not mean for them to be twins.

EDIT:
Len's voice is not altered in any way. Asami Shimoda was hired because she is able to create both male and female sounding voices. Len is Asami's raw voice without any editing :]

She sings as both Rin and Len in Rin Rin Signal ( link )

there's really no official explanations. some people prefer calling the two of them "mirror images of each other"--but since they're not exactly "identically mirrored images", they can be considered twins (brother and sister) as well.

i doubt thaey are caues they make so many love songs togather

Well, Rin and Len can be brother and sister. Also they can be a couple. See, the people who created them did not give us an obvious answer. But, they let the fans choose if they are brother and sister or a couple. Personally i think of them as a couple because of all the love songs they created. Ex. magnet,spice ext... But you can also think of them as a couple by the songs, Servant of evil, daughter of evil ext... it's up to you. :)

No they are not! Crypton made them as "mirrored images". They did not officially say if they were twins, lovers or siblings. They left it the way it was so as not to ruin everyones creativity. It is the fan's decision to pick what they are.

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Rin x Len 4ever

Yes they are because the same last name and they are twins

Len and Rin are actually mirror images as everyone are saying. Also the creators of VOCALOID (crypton) decided to leave rin and len up to the users to decide what they could be in there video . they could be lovers,twins,ect. The last name "kagami" does mean "twin"... just search it up,it will come. X3

Actually, I've heard that the creators of them said they were originally mirror images of eachother, but fans can interpret them as whatever they want. (twins, lovers, etc.)

Len is actually the mirror image of Rin so,or what.

They were originally mirror images. But then they were boyfriend and girlfriend, and then twins. It's okay to refer to them as brother and sister, since they are, but they are also the other things I listed. It's confusing...

No,they are mirror images.

It depends on the song they are singing
Example:
In their song adolescence they are lovers

As you people can see.. about 70% Rin Len songs has them as lovers, 20% as twins and 10% as mirror images.. but this doesn't make them any in particular.. Mirror images is the main concept.. while fans can think of them in anyway they want...

They are TWINS DON'T TELL ME THERE F***ING MIRROR IMAGES!!!-
And there not lover's eaither sorry to burst you're bubble... of course I have always thought of Ishiko and Len to be a couple... Idk why....

well in my opion there lovers they may seem like twin but to me there just plain lovers they sing a lot of love songs and they seem like the perfect match that's my opion anyone can deal wth that -_-
Sugarcube OUT

Yes crypton intended them to be twins but they are actually mirror images hence their last name Kagamine. Kagami meaning mirror and ne being sound. The reason I think is to give the fans that choice but also the fact that in some songs they're lovers and it'd be awkward if they're siblings.

yes!!!

brother and sister they are not lover they are twin you can see by there eye color.

They can twins. Or lovers.
Lovers song: Cantarella
Twins song: Electric Angel