Watch My Sister's Serial Killer Boyfriend Full Movie Free Online 123Movies. Maddie, a local reporter investigating a series of killings targeting similar looking women is caught off guard when her younger sister Olivia, is attacked in her own home. Urging on the side of caution, Maddie suggests they take a self-defense course. When Olivia starts to fall for their instructor, Maddie becomes skeptical when he refuses to reveal anything about himself, and tries to intervene before the relationship turns into a deadly obsession.