Younger sister - connected to the death of her mother, was the object of comparison with her sister ( she was told not as smart, not as nice and well mannered), loss most of her memories thus struggling in life and school, was taken into the Goblin world in attempt to find her sister, in the goblin world she was chased by demons and goblins who were trying to kill and eat her, still her main point of motivation was to find her sister, met all of the princes and had trouble with them (except the third - have not met him yet - I am up to chapter 29), was lied to and tricked by the second prince, was turned into a half Goblin (making her have a lower chance of leaving), and yet still trying to escape to save her sister.
The older sister - took the responsibility of a mother of her own free will, was taken into the goblin world and was in the hand of the third prince, but decided to stay because she felt sorry for him, thus not trying to escape.
The journey of the sisters
Younger sister - connected to the death of her mother, was the object of comparison with her sister ( she was told not as smart, not as nice and well mannered), loss most of her memories thus struggling in life and school, was taken into the Goblin world in attempt to find her sister, in the goblin world she was chased by demons and goblins who were trying to kill and eat her, still her main point of motivation was to find her sister, met all of the princes and had trouble with them (except the third - have not met him yet - I am up to chapter 29), was lied to and tricked by the second prince, was turned into a half Goblin (making her have a lower chance of leaving), and yet still trying to escape to save her sister.
The older sister - took the responsibility of a mother of her own free will, was taken into the goblin world and was in the hand of the third prince, but decided to stay because she felt sorry for him, thus not trying to escape.