Mary Zahau-Loehner said she found investigators unconvincing during a visit to her home Wednesday in St. Joseph, Mo., to break the news.
"It doesn't add up," she told The Associated Press. "Nothing adds up."
San Diego County Sheriff's Department spokeswoman Jan Caldwell declined to comment. Sheriff Bill Gore planned a news conference Friday to discuss investigators' findings.
Rebecca Zahau, 32, was found dead July 13 at the mansion in suburban Coronado, two days after a 6-year-old boy under her care was seriously injured in a fall down the stairs. Max Shacknai later died.
Both were linked to Jonah Shacknai, an Arizona pharmaceuticals magnate who owns the 27-room waterfront mansion. Zahau was his girlfriend of two years. Max was his son from a marriage that ended in divorce in 2008.
Zahau-Loehner said investigators told her they found no suicide note. They did share text messages on her phone from months earlier about "issues" between her and Shacknai's children.
Zahau-Loehner said she spoke with her sister the night before her body was found and she gave no hint that she planned to take her life. She said she planned to bring Jonah Shacknai breakfast and a change of clothes the next morning to the hospital where his son was being treated.
Zahau also told her sister that she would call her parents in the morning on the way to the hospital. She emailed another sister in Germany to say she would be updating throughout the next day.
"Too detailed planning for someone who's planning to end their life that night," Zahau-Loehner said.
When Max fell down the stairs, Zahau was at home with a 13-year-old girl who was related to her, authorities say. He was hospitalized after paramedics found the boy wasn't breathing and did not have a pulse.
Two days later, Shacknai's brother, Adam, called 911 to report that Zahau appeared to be dead, investigators said. The brother was staying in a mansion guesthouse. Jonah Shacknai was not at home.
Investigators will also announce their findings on the boy's death Friday. They initially said they believed the fall was an accident.
The home, known as the Spreckels mansion, is one of the more storied properties in Coronado, a small suburb of multimillion-dollar homes on the tip of a peninsula across a bay from downtown San Diego. The home has unobstructed beach views and sits near Coronado's main street, which is lined with palm trees and upscale boutiques.
Shacknai bought the mansion in March 2007, when it was assessed at $12.75 million. The home was built in 1908 and named for its original owner, John D. Spreckels, who also owned the newspapers, the San Diego Union and San Diego Tribune.
Shacknai founded Medicis Pharmaceuticals Corp. and has been chairman and chief executive of the Scottsdale, Ariz.-based company since 1988. In a divorce filing, he filed a tax return that showed his income in 1998 at $17 million.
Medicis makes acne treatments Solodyn and Ziana and facial wrinkle treatment Restylane and Dysport, a competitor of Botox. Last year, the company earned $123 million on $700 million in revenue.
Shacknai has two children with his first wife, Kimberly. Max was his only child with his second wife, Dina.
He told Medicis investors on a conference call Aug. 8 that he was planning to resume his daily routine of showing up at the office as soon as school resumed for his children in Arizona.
"We've obviously had an extraordinarily difficult time. We have undertaken some tragedies, some losses that one couldn't imagine experiencing in a lifetime," he said.
Zahau, a native of Myanmar, was an ophthalmic technician at Horizon Eye Specialists & Lasik Center in the Phoenix area from April 2008 to December 2010.
Zahau-Loehner urged investigators keep the investigation open and said the family has hired an attorney.
"There are many unanswered questions," she said.