What is the English word for the Swahili word ndugu?

Sibling is one meaning.

In general it means relative or family member, and is used to apply to all imediate and distant relatives, as well as figuratively to close friends. In Tanzania when Mwalimu J.K. Nyerere was president, it was common to address both men and women a , e.g., Ndugu Juma or Ndugu Miriamu in the sense of Mr. or Ms. or Comrade. Many people still use this address; the President is widely called Ndugu Kikwete.