Amnon was the progeny of David and Ahinoam. Tamar and Absalom were the progeny of David and Maacah. Thus Amnon and Absalom were half-brothers, like Abraham's son's Ishmael and Isaac.
Jewish law forbade Amnon to marry a half-sister, and taking evil advice he pretended to be sick. He lured Tamar into his quarters with the pretense that he wanted her to cook a special meal for him, ignored her protests and "lay with her". Two years later, Absalom engineered Amnon's death.