I think there are two timesplits of the west woods, so whatever one you find canon and true, that is what is correct. In my case; Sliverstone existed 100%, it's just that Robinwing and the other Fireclan cats (except for Breezestar) believe that Littlebird did kill Firestar and not Sliverstone.
Destiny was Sliverstone when she retired to Nightmare. Crystalblaze tells the player that they traveled together as a kind of figurative language meaning "I lived alongside her and always was in Brambleclan, but she left for the barn. "
