Volvie, There’s nothing there about how it makes as much sense to say the Sun goes around the Earth as vice versa.
The whole point of the theory is that there is an absolute frame of reference. The speed of light is a constant no matter what you are doing. The difference between Special Relativity (SR) and General Relativity (GR) is that the former holds in inertial, non-accelerating frames of reference. GR unifies SR and gravitation as well as a number of other fascinating mathematical advances. SR is very well established. GR still quite a bit less so.
Any high school physics student can tell you the Earth and Sun both rotate around their common center of gravity which is located an infinitesimal fraction away from the center of the Sun.
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In GR all reference frames are equally valid. This is derived on a non-inertial reference frame.