Authorities have ruled Rebecca Zahau's death a suicide. They believe she felt responsible for the death of her boyfriend Jonah Shacknai's son who she was babysitting.
The boy died after tumbling down the stairs while in her care.
Zahau's sister spoke exclusively to Good Morning America's Elizabeth Vargas and she says it just doesn't add up.
Vargas: She was found not only nude but with her hands tied behind her back?
Mary Zahau: Correct
Vargas: And her feet tied. Why would someone do that?
Mary Zahau: I had the same question. It seemed like from the beginning a lot of things were ignored.
Vargas: So if you don't think your sister committed suicide, you're saying your sister was murdered?
Mary Zahau: Yes.
In a statement, Shacknai called Rebecca Zahau, "a wonderful and unique person who will always have a special place in my heart."