In a statement released online, Sakinab Shah said 53-year-old captain Zaharie Ahmad Shah was a "man of integrity" and that he had been acting normally when they last met about two weeks before the Malaysia Airlines plane went missing on 8 March last year.
Speculation had emerged shortly after the plane disappeared that Zaharie had suffered some personal problems, and more recent reports have suggested that pilot action remains among the most plausible explanations for the mystery.
"As things stand today, with no tangible evidence to show, NO ONE, be you politician, scientist, aviation expert, plane crash investigator, pilot, retired pilot, media or whoever else... NONE OF YOU have a right to blame Captain Zaharie Ahmad Shah for any wrongdoing," the posting which surfaced this week said.
"I want the world to know here is a loving man who will stop at nothing to render help when it is needed."
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Peter Chong, a close friend of Zaharie and his family, confirmed to AFP the posting was genuine.
The plane, carrying 239 passengers and crew, swerved off of its route from Kuala Lumpur to Beijing for reasons unknown. No trace has been found despite an ongoing search in the southern Indian Ocean where experts believe it eventually crashed.
Malaysian authorities investigated the cockpit crew's backgrounds and examined a flight simulator that Zaharie had used at home, but have said nothing suspicious ever turned up.
It isn't the first time that relatives have sought to defend the reputation of Zaharie, a 33-year veteran with Malaysia Airlines who was highly regarded by his peers.
While his family has avoided the media glare, both his son and daughter last year separately defended him in public statements.
"He certainly didn't have the right foundation to be able to figure out a scheme to hijack his own plane and then to disappear into thin air without leaving a trace," she said.