Conciousness is the ability to model the self, make predictions about one’s own behavior in response to various environmental conditions, and improve one’s personal outcomes by first modeling, and then acting based on the predictions of the model. The model actually can model itself, as well.
Sentience, I think, is about consciousness but looking from the outside. If we see behaviors that indicate that another life form is modeling it’s own sense of self, then I think we would believe it is sentient.
Of course, both these things exist on a spectrum, and I would not hazard a guess as to where on that spectrum one could draw the line between consciousness and non-consciousness, or sentience and not sentience.