According to the theory of special relativity, the situation you described involves time dilation, which is the phenomenon where time appears to pass at different rates for objects moving relative to each other.
In the scenario of the astronaut traveling in a spaceship at 0.99 times the speed of light (0.99c), if we consider the astronaut's reference frame, it is true that the twin on Earth appears to be moving away from the astronaut at 0.99c. However, when the astronaut returns home and compares their clocks, they will find that the twin on Earth has aged more than they have.
This might seem counterintuitive, but it can be explained by considering the relative motion between the two frames of reference. In the astronaut's reference frame, they are at rest, and it is the twin on Earth who is in motion. According to the astronaut, the twin's time is dilated due to their high velocity relative to the astronaut.
From the twin's perspective on Earth, it is the astronaut who is moving away from them at 0.99c. The twin also sees time dilation in the astronaut's frame, which means that the astronaut's time is passing more slowly compared to their own.
The key point to understand is that both observers—the astronaut and the twin on Earth—will perceive the other's time to be dilated. However, when they compare clocks at the end of the journey, they will find that the twin who experienced acceleration and changes in inertial frames (the astronaut) has aged less than the twin who remained in a single inertial frame (the twin on Earth).
This is a consequence of the fact that the astronaut's frame of reference involves acceleration and changes in inertial frames, while the twin on Earth's frame is inertial throughout. The twin on Earth ages more because they experience less acceleration and remain in a single inertial frame for the entire duration of the journey.
So, in summary, the twin paradox is resolved by considering the different frames of reference and the acceleration involved, leading to the conclusion that the twin who traveled at high speed ages less than the twin who stayed on Earth.