Step 1: Understanding the Seven Sisters
- The Seven Sisters of Northeast India include Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura.
- These states are collectively known for their geographical and cultural uniqueness.
Step 2: The role of Sikkim
- Sikkim is not geographically counted among the Seven Sisters.
- However, due to its proximity and strong cultural, social, and economic ties with the Northeast, it is popularly called the “Brother” of the Seven Sisters.
Step 3: Analyzing the options
- (A) Sikkim: Correct, called the brother of the Seven Sisters.
- (B) West Bengal: Incorrect, though it connects the Northeast via the Siliguri Corridor, it is not called the brother.
- (C) Assam: Incorrect, Assam is itself one of the Seven Sisters.
- (D) Manipur: Incorrect, also part of the Seven Sisters.
Step 4: Conclusion
Thus, the state called the brother of the Seven Sisters of Northeast India is Sikkim.
\[
\boxed{\text{Correct Answer: (A) Sikkim}}
\]