The concept of relative motion does not imply that all perspectives or statements about motion are equally valid. While it is true that motion is relative in the sense that the motion of an object can be described with respect to a particular reference frame, some reference frames are more useful or appropriate than others depending on the situation.
In the case of the Sun, Earth, and their motions, it is more appropriate to say that the Earth revolves around the Sun rather than the other way around. This is because the motion of the Earth around the Sun provides a simpler and more accurate explanation for the observed phenomena in our solar system.
From a heliocentric perspective (Sun-centered frame of reference), the motions of the planets, including Earth, can be described using simple laws of planetary motion, such as Kepler's laws. These laws allow for accurate predictions of the positions and behaviors of the planets.
On the other hand, from a geocentric perspective (Earth-centered frame of reference), the motion of celestial bodies becomes far more complex and requires additional assumptions, such as the introduction of epicycles, to account for the observed motions. This geocentric model is significantly less accurate in predicting celestial phenomena.
Throughout history, as our understanding of the cosmos improved and more precise observations were made, the heliocentric model gained prominence because it provided a more elegant and accurate description of the motion of celestial bodies, including the planets.
While the principle of relativity acknowledges that different observers can have different perspectives on motion, it does not imply that all perspectives are equally valid or accurate in explaining the underlying physical phenomena. In the case of the Sun and Earth, the heliocentric model provides a more comprehensive and accurate understanding of their motions in the solar system.