Is there a reaction pair for $mg \sin \theta$ on a inclined plane?
It is there for the normal force on a block on a flat plane, so is it there for $mg \sin \theta$ also?
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$mg\sin\theta$ is the component of the gravitational force which is directed down the ramp. It's exerted by the Earth on an object which is on the ramp. The reaction force is the equal and opposite gravitational force exerted by object on the Earth.
Force pairs are always interactions between two objects. So you can always say that the force exerted by object A on object B is equal and opposite to the force exerted by object B on object A. There are no exceptions.