Sci-Fi and fantasy share a lot of DNA, and were, at one time, not distinguished as genres, but rather both taken up under the umbrella of romances. That was a long time ago though, and they are currently fairly well defined as separate genres. That said, the common core they share is well reflected in the amount of genre bleed you see, with elements of one crossing over to the other fairly regularly in specific IPs. There's not a whole lot of definitional oomph in trying to define them as entirely different things.