The Kindred (1987)

"John's got a half brother... half human... half something else!"

Amanda's deathbed request to her son, John, was for him to destroy all the lab notes etc. from her last experiment. She also blurts out he had a brother. At the funeral John meets Melissa, who claims to be his mother's biggest fan. Together with some of John's friends they go to Amanda's house, but none are prepared for what they find there.