They`re Mirror প্রতিমূর্তি of themselves , but আপনি can say they`re twins . cause twin = 2 . and yeah . অনুরাগী think of them as twins.
KEI stated in an interview that he specifically did not নকশা them as twins, and was told the two are আরো like mirror প্রতিমূর্তি of eachother and নকশা them as such দ্বারা CFM. He ব্যক্ত since he didnt নকশা 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 )