A group of college friends consisted of Harvey, Nikki, Rob, Skip, Nan, Chaz, Kit, and Archâgather, celebrate spring break by spending the weekend at the island mansion of their friend and Skip's sister Muffy St. John, on the weekend leading up to April Fools' Day. The tone is set almost immediately with Muffy preparing details around the house, and finds an old jack-in-the-box she remembers in a flashback sequence. Meanwhile, her friends are joking around on the pier, then on the ferry to the island. When en route to the island, local deckhand buck Buck is seriously injured in a gruesome accident.
Once on the island and in the mansion, it turns out Muffy has set up a variety of little jokes from the simple such as whoopie cuchions and dribble glasses, to the more complex and disturbing such as an audiotape of a baby crying in someone's room and heroin paraphernalia in a guest's wardrobe. Through it all, despite the friends in general try to relax, Skip goes missing. Kit catches a glimpse of what looks like his dead body. Soon, Arch and Nan also go missing next. During a search, Nikki falls into the island's well and finds the severed heads of Skip and Arch and the dead body of Nan. Afterward, the remaining group discover that the phones are dead and there is no way to get off the island until Monday.
One after another, the group either vanishes or being killed and their bodies being found. Kit and Rob put together some clues and realizing that everyone's earlier assumption is wrong; the kinsman of the deckhand injured when they arrived is a red herring. It also turns out that Muffy has a violently insane twin sister named Buffy, who has escaped. In fact, the "Muffy" they have been around since the first night was Buffy, pretending to be Muffy. They discover Muffy's severed head in the basement.
Buffy chases them with a curved butcher's knife, and the couple are separated. Kit flees from Buffy into the living room where she finds everyone else there, alive and calmly waiting for her. It was all a joke, or more accurately, a dress rehearsal. It is revealed to the audience that the whole film was never a slasher film from the start, but rather pretending to be one. Muffy hopes to turn the mansion into a resort offering a weekend of staged horror. She even had a friend who does special effects and make-up for Hollywood help. Each "victim" agreed to take part as things were explained to them.
Everyone has a huge laugh and break out lots of bottles of champagne. Later that night, a half-drunk Muffy goes to her room and finds a wrapped present on her bed. She unwraps it, and the present is the Jack-in-the-Box. Savoring the surprise, she turns the handle slowly and when "Jack" finally pops out, Nan, who knew Muffy from acting class, emerges from behind her and slits her throat with a razor. Muffy screams, but then realizes she is not really bleeding and that she used a trick razor and stage blood. The film ends with the Jack-in-the-box winking at the audience.

Question: Who has been pretending to be Muffy? 
Answer: Buffy, her violent twin sister
Alternative answer: Buffy