Explanation:

The most effective word to fill in the blank is option 'c) different'.

Reasoning:

Twins are siblings who are born at the same time and have a similar genetic makeup. However, even though twins share many similarities, they can still have differences in their appearance, personality, and other characteristics.

In the given sentence, it is mentioned that the two sisters are twins, indicating that they were born at the same time and have a similar genetic makeup. However, the word 'somewhat' suggests that there are differences in their appearance.

Let's analyze the other options to understand why 'different' is the most effective word to fill in the blank:

a) 'alike': This word means similar or having a resemblance. While twins can be alike, the word 'somewhat' indicates that there are differences between the two sisters. Therefore, 'alike' does not accurately describe the relationship between the sisters in this context.

b) 'unique': This word means one of a kind or having distinct characteristics. While each individual is unique in their own way, the word 'somewhat' suggests that the sisters have some similarities but also some differences. Therefore, 'unique' does not accurately convey the idea of similarity with some differences.

c) 'different': This word means not the same or having dissimilar characteristics. In the given sentence, the word 'somewhat' indicates that the sisters have some differences in their appearance. Therefore, 'different' accurately describes the relationship between the two sisters in this context.

d) 'related': This word means connected by blood or marriage. While the sisters are related as twins, the word 'somewhat' suggests that there are differences in their appearance. Therefore, 'related' does not accurately describe the idea of having some differences.

Conclusion:

The word 'different' is the most effective word to fill in the blank as it accurately conveys the idea that although the two sisters are twins, they have some differences in their appearance.