I think they're human beings and not a matching tea set.
IMO twin names should definitely have some common elements. Both or neither having a nickname, similar length, similar popularity, similar level of fanciness. But making them that matchy just drives home that each girl will be thought of as "one of a set" and not her own person. And people already find it difficult enough remembering which sibling is which if they don't see them often, especially twins.
Besides,
Siri is so heavily associated with the iPhone PA that it would be weird to see it on a person.
What about
Iris and
CecileIris and
JaneIris and
LydiaIris and
MabelIris and
MaudIris and
SylviaIris and
Una