Lavinia stands at the doorway of the Mannon sitting room, her hair, dress, and movements identical to Christine's. The room has long been shut up and its furniture covered. She pauses beneath the portraits and addresses them resentfully: wondering why they stare at her.
A dazed Orin appears. He expected to find Mother in the study, but she is nowhere. Now she will never forgive him. In any case, he is no longer her son; he is a Mannon now and the house will welcome him. Lavinia harshly orders him to stop his rambling and then moves to sooth him. They must get back to normalcy and begin a new life with Peter and Hazel. They have every right to their love.
To Lavinia's shy delight, Orin grimly remarks on her new resemblance to Mother. Since they sailed East, she has been stealing Christine's soul. Death has set her free to become her. Lavinia rebukes her brother; he promised at the Islands that if he came home to face his ghosts, he would rid himself of his morbid spells and guilt over the past. Orin replies maliciously that it was his brotherly duty to get her from the Islands. Lavinia forces Orin to repeat after her: mother killed father, and they only did what was just. They set themselves to cleaning the house.
Peter enters from the rear and gasps, thinking he has seen Christine's ghost. Lavinia approaches him with eager possessiveness. Peter is especially stunned to see Lavinia out of black; Lavinia replies that she was dead back then. Orin mocks his sister, accusing her of stealing Mother's colors and becoming a true romantic while under the influence of the Islands. Indeed, the Islands and their men in particular turned her into a regular pagan. Another month more and Lavinia would have joined the natives naked under the palm trees.
An angry Lavinia forces a smile. She straightens Orin's clothing and sends him off to Hazel. When she criticizes his rigid posture, Orin cunningly retorts that she'd rather he play the clipper captain than Father. Orin departs.
Lavinia declares her love for Peter and warns him against falling for of Orin's morbid spells. She did not flirt with any of the native men. The Islands did finish setting her free, but her time among natives and ignorant of sin enabled her to forget death and come to love and beauty. Now she and Peter will marry, leave the region, and make an island for themselves in the country. They embrace. Suddenly Hazel and Orin appear in the doorway. Orin starts in a jealous rage but Lavinia commands him to be still. She stares at her brother in dread.
In Mourning Becomes Electra you write: "Oedipus was the Theban king who unwittingly killed his father and MURDERED his mother." [Emphasis mine].
It should read: "Oedipus...MARRIED his mother!"
(Oedipus' mother Jocasta did commit suicide after learning her lover was her son. Oedipus however did NOT "murder" her.)