The paradox isn't actually when the twin is returning, the paradox is seen during the trip away from earth. Both are observing each other through telescopes, and both can see the clock on the wall in each other's home (a house, or for the twin, his space ship. Which has got a big window). And both can observe that the other twin's clock is moving at a slower pace than their own clock. And that's the paradox.

(p.s. the spacefaring twin doesn't have to move at the speed of light, and indeed cannot, it's enough to move at a "relativistic" speed, i.e. fast enough that this is actually measurable. With today's clock that doesn't have to be very fast actually)