Before Vocaloid 3 there were no official Vocaloids that were siblings. This changed with the release of i-Style Project's Vocaloids, Aoki Lapis and Merli, they are officially sisters.
If other official Vocaloids had siblings, they were fanmade characters, these are characters created by Vocaloid fans. And besides Rin and Len who are mirror images of one another, yet can be looked upon as twins.
- Sonika has called Leon, Lola Miriam her older siblings. This is open to interpretation though as siblings can be taking literally or metaphorically. See Tweet here.