Introduction:
The two sisters referred to in the riddle live in twin towers. One sister is fair and the other is dark. Additionally, one sister is from the sea while the other is from land. Let's analyze the riddle and try to identify who these sisters could be.

Analysis:
1. Twin Towers: The mention of twin towers suggests that these sisters live in two separate buildings that are identical or very similar in appearance.
2. Fair and Dark: One sister is described as fair, while the other sister is described as dark. This could refer to their skin color or hair color.
3. From Sea and Land: The riddle mentions that one sister is from the sea while the other is from land. This implies that one sister resides near the ocean or sea, while the other sister lives inland.

Interpretation:
Based on the given information, we can conclude that the two sisters in the riddle are lighthouses. Let's break down the analysis and explain why lighthouses fit the description.

Explanation:
1. Twin Towers: Lighthouses are commonly built as tall structures with a tower-like appearance. They often come in pairs, especially in areas with treacherous coastlines or busy shipping routes.
2. Fair and Dark: In this context, the terms "fair" and "dark" refer to the lights emitted by lighthouses. Lighthouses have distinctive light patterns that help ships navigate safely. The fair sister represents the light with a shorter duration, while the dark sister represents the light with a longer duration.
3. From Sea and Land: Lighthouses are typically built near coastlines or on islands, making them close to the sea. This aligns with the description of one sister being from the sea. On the other hand, there are also lighthouses located inland, often near bodies of water such as lakes or rivers. This corresponds to the sister from land.

Conclusion:
In conclusion, the two sisters in the riddle are lighthouses. They live in twin towers, emit different light patterns (fair and dark), and are located near the sea and land respectively. Lighthouses play a crucial role in maritime navigation, providing a visual aid to ships and ensuring safe passage.