Roman doesn't die in Brown Girl Dreaming. He does, however, get extremely ill.
Roman is the youngest child of the family; his birth meant that Jacqueline was no longer the baby. However, when he eats lead paint off the walls and has to be hospitalized, Jacqueline misses him. She suddenly doesn't mind that she's not the baby. She just wants the real baby back in the house. Her issues with Roman caused her to resent him in the past. She even pinched him after he was born.
Lead poisoning is a very serious illness and can cause issues with development. This is why Roman is gone for so long. He wants to travel with his siblings, but he has to stay at the hospital. The other kids promise to bring him toys and treats when they return. Jacqueline says he's small and his siblings don't understand why he has tubes from his arms, why he's so small, and why he sleeps so much.
He comes home from the hospital when he's four.