Worf’s brother in Star Trek: The Next Generation is Kurn.
Kurn is Worf’s younger biological brother and a member of the House of Mogh. His identity is revealed when he serves aboard the Enterprise as part of a Klingon officer-exchange program. Kurn specifically requests the assignment so he can observe Worf and learn how his brother has adapted to life in Starfleet.
Their family history is shaped by the destruction of Khitomer. After their father, Mogh, is accused of betraying the Klingon Empire, Worf accepts ritual discommendation to prevent a civil war between the House of Mogh and the powerful House of Duras. Kurn’s identity is initially kept secret so that the family’s remaining honor and political position can be protected.
Later, Kurn fights beside Worf during the Klingon civil war and helps Gowron defeat the forces loyal to the House of Duras. Gowron restores the family’s honor, allowing Kurn to take a seat on the Klingon High Council. Kurn is not Worf’s son: Alexander is Worf’s son, while Duras is a rival Klingon leader.