The girls are Caitian just like Jadzia Dax is Trill.
Nah, the basics of Trills remained the same: the symbiont. The bumpy forehead used in one TBG episode didn't look good on DS9's permanent cast member, so they switched latex bumps for spots rather than put their most attractive female cast member under daily latex for six years.
While the twins in STiD do have cat tails (like those of Caitians), it seems the final facial makeup used onscreen doesn't even match up with that described by the actresses in their interview. There doesn't seem to be anything felinoid about smooth torso skin, face scales and cheek bumps, and the alien women don't seem to have latex felinoid ears nor latex felinoid noses.