tin nyanko do have a bit of a relationship in the anime and the manga, but especially in the anime. for one, they are all cats!!! (well, sort of.) and in the manga, they all came from the same planet: mau. and the manga said that nyanko was originally intended to help luna and artemis as guardians of the sailor senshi, but the galaxia came along and turned her evil. grr!!! so when diana goes to the past, she sees nyanko facing off with luna and artemis (when they are humans). nyanko accused them of being traitors and leaving mau. so then she tried to blast luna, artemis, and diana with her galactica puppet attack, but eternal sailormoon came along and blocked it. but eventually, nyanko was able to scratch all three of their moon symbos, turning them into cats.
in the anime, they never really say anything about nyanko being from the planet mau, but of course she collects the star seeds so that she can one day be sailor senshi mau, so i figure that they just forgot. ^^b