Govt employee here who had also cracked one govt job of Dept of Atomic Energy.
The person calling you is from the Intelligence Bureau. You brother must have mentioned about you in his Attestation and Special Security Questionnaire.
This is a routine process in which apart from the local PS, the IB also conducts verification to ensure that the perspective Govt employee (your brother here) has no such links which may put national security at stake.
This IB verification is usually done for sensitive depts like ISRO, DRDO, DAE etc