Twins paradox

In Einstein's  theory of "special relativity", there's a phenomenon called "time dilation". In Einstein's theory, time is not singular. It passes differently to different observers. For the observer who mover extremely fast(close to the speed of light), his or her time passes much slower than the observer who stands still. One of the examples for this theory goes that: There are two hypothetical twins. One stays on Earth while the other travels in space with an ultra-fast rocket, nearly as fast as the speed of light. Two years later(when the twin who travel think that two years have pasted), the twin who traveled in space returned to Earth and found that his brother has aged 30 years while he only aged 2 years. This thing happened because in the observation of  the twin on Earth, the twins on the spaceship traveled in a very high speed so that the twin on the earth feel that the time on the spaceship goes much slower.Therefore, a big differences of time caused. However, here's a problem. As we know that motion and speed are relative, we can also say that the twin on the spaceship felt that his brother travel in the speed of light with earth and went away from him, so he would feel the time on the earth goes slower and he would age more than his brother on Earth. Therefore, we can't decide which twin's time goes slower or their time goes in the same pace. To solve this problem, we have to decide that in which condition does time go slower, and for the dictum "Moving clocks go slower '' to hold, you must be an inertial observer. To be an inertial observer, one must stay in a reference frame on which every force apply are perfectly balanced or a frame that is not influenced by any force. To put it simply, the net force apply on the inertial frame is zero, so it either travels in a constant speed or stays still. In the twins paradox, both the Earth and the spaceship are not inertial frame because Earth moves around the Sun and the spaceship was in the process of accelerating and decelerating when it moved away from Earth and then returned. However, compared to the spaceship, the influence of the motion of Earth is small, so we can take the observer on the Earth is inertial observer. Hence, the law "moving clocks go slower" can be applied to the observers on Earth. For the observers on Earth, spaceship moved away from them, and they thought of themselves as being still. As a result, observers on Earth would feel that the time on the spaceship went much slower. When the travelers came back, observers on Earth aged more than the travelers.