Twins are fairly common for among sprites and other 'produced' nekos (that is, ones who are grown in a hemosynth tank instead of born), in my experience. See Ichigo, Nimura, Peppermint, and Cinnamon on the wiki. Probably several other pairs that I'm forgetting due to the fact that it's 3:30 AM. Dunno if them being born instead of grown was integral to your idea or not.