Nope - you thought correctly. Rimururu is definitely younger than Nakoruru. They're actually not even related by blood...which totally eliminates any possibility of them being twins. I've removed the twincest tag, as a result.
I'm inclined to think it's Nakoruru's two selves, Chivalry and Treachery (better known as Slash and Bust), judging by the other's skin tone, white and purple dress (as opposed to Rimururu's blue), brown hair and shorter hair length. I'm not certain which of the SS games this design was used in, however... IV perhaps? The selfcest tag would be appropriate.
