Weirdly (at least imho) the twins were supposed to be
Caitians which I don't buy myself. They seem vaguely feline, but very different from actual Caitians we've seen before in TVH and TAS. But then I tend to reject statements of intent by designers, etc. as canon until it ends up on screen somewhere. Recall Andorian antennae used to be what that species used for ears!
So I think of the twins as a different alien species. Their look is cool enough.