Not a stupid question at all Fergie!
So it really depends on if your Sky TV set-top box has an HDMI out on it. If it does, then you just go:
Sky TV -> Xbox One -> Receiver -> TV
The catch here is that while you are gaming you have to snap TV in, which would make the screen a little smaller to snap in the TV app. And you do bring up an interesting question. Can you toggle which device outputs sound, or does it mix it together? I doubt I'll get an answer but I'll tweet a couple people and see if I can find out.