Kabhi Khushi Kabhie Gham

Yashvardhan "Yash" Raichand (Amitabh Bachchan) is a rich businessman living in India with his wife Nandini (Jaya Bachchan) and two sons Rahul and Rohan. Rahul is the elder son and was adopted by Yash and Nandini at birth. This is known to everyone in the Raichand household, except Rohan. When grown up, Rahul (Shahrukh Khan) comes across the vivacious Punjabi-speaking Anjali Sharma (Kajol), and they eventually fall in love, but their love is forbidden, because Anjali is from a working-class background. Yash and Nandini soon send Rohan to a Boarding School, which all the males in their family have attended. Yash announces his desire for Rahul to marry Naina (Rani Mukherji), but Naina learns that Rahul is in love with Anjali, and encourages him to pursue her. When Yash comes to know of this, he is enraged, and Rahul promises not to marry Anjali. En route to tell her of his promise, he discovers that her father (Alok Nath) has died, and decides to marry her despite his father's hostility. Yash learns of the marriage and disowns Rahul, whereupon Nandini and Rahul share a tearful goodbye. Nandini sends Sayeeda (Farida Jalal), Rahul and Rohan's nanny, to watch over him. Rahul visits Rohan in the Boarding School and begs of Rohan never to ask where he went or why he left, and asks that he take care of Nandini.
Ten years later, Rohan (Hrithik Roshan) returns home from the Boarding School, and learns from his grandmothers (Achala Sachdev and Sushma Seth) that Rahul was adopted, and the reason why Rahul left the house, whereupon Rohan vows to reunite the family. He learns that Rahul, Anjali, and her younger sister Pooja moved to London, and tells his parents that he wishes further studies in London, his parents agree to let him go. In London, Rahul is now a wealthy businessman. He and Anjali now have their own son, Krish. Pooja (Kareena Kapoor), now a popular fashion-obsessed diva, is a student at King's College London. Rohan enrolls there too. He and Pooja meet again and he tells her that he has come to London to bring his brother and sister-in-law back home. Pooja tells Rahul that Rohan is her friend's brother who currently has nowhere to stay. After some persuasion, Rahul agrees to let Rohan stay. At Krish's school function, he leads his class in singing the Indian National Anthem, and later recites advice which Rohan had given him. Rahul having given Rohan this advice ten years back, he now realises that Rohan is his brother.
Rohan begs Rahul to come home, but he refuses. Pooja convinces Rohan to invite his parents to London; Rahul and Nandini are overjoyed to see one another, but Rahul still refuses to talk to his father. Soon, Yash learns that his mother is on her death bed, and that her last wish is for Yash, Rahul, and Rohan to light her funeral pyre together. Therefore, the entire family attends the funeral, and Nandini tells Yash that she thought he was wrong for disowning Rahul. Rohan and Pooja convince Rahul to talk to Yash, who allows him and Anjali into the house. Rohan and Pooja are married and the family hold a belated celebration of Rahul and Anjali's wedding.

Question: How are Rahul and Rohan related?
Answer: Brothers