Layla isn't the Prismriver's sister but master they were modeled after Laylas sisters who disappeared besides even if the real lunasa did come she would be dead as well as she is older than Layla
Suppose that's not Layla's grave. Suppose it's one of the other Prismrivers... the real ones, not the deathless shades Layla created out of loneliness. Suppose it's the grave of Lunasa's own predecessor.