The latent content of your dream would be wishing she were dead, while the manifest content would be pushing her off the cliff.
Latent means hidden, unconscious - you don't really wish your sister to die, it is just something that came up because you were angry. However, now that feeling is there, it manifested itself in your dreams (according to Freud, our unconscious desires become partly conscious in dreams) when you pushed her off the cliff in order to kill her.
