Speaking publicly for the first time about her sister's death, Ami Denborg told the London-based newspaper The Times that her family had been charmed by the wealthy Shrien Dewani.
South African authorities accused the 30-year-old businessman of paying to have his bride Anni killed in a hijacking during their honeymoon in Cape Town.
Speaking from her home in Sweden, Denborg, 32, would not say whether her family thought Dewani played a part in her sister's killing.
However, if he was found guilty, "then what he has done is unforgivable. You can't just kill somebody. It is scary. What the hell was he thinking?" she told the paper.
She said the family would go no further on the matter until the legal process was completed and they felt justice had been served.
"It is terrible enough to lose a sister but it is even more terrible to lose a sister in such a way," she said.
"The most sad part in all of this is that it doesn't matter what happens to Shrien, to the driver, or to whoever killed her - I will never get my sister back."
Dewani appeared on Wednesday at City of Westminster Magistrate's Court in London on an extradition warrant.
Dewani had handed himself in to a police station in Bristol, southwest England, late on Tuesday.
His 28-year-old wife was killed on November 13 after the couple's taxi was reportedly hijacked outside Cape Town.
In the Western Cape High Court on Tuesday, taxi driver Zola Tongo said he was offered R15 000 by Dewani to kill his wife.
Dewani strongly denies any involvement in the murder and told the court in London he did not agree to the extradition.
The judge in London first granted bail to Dewani, but in a dramatic twist he was told that for procedural reasons he must now remain in custody pending a High Court hearing.
District Judge Howard Riddle said initially he had agreed to grant bail to Dewani because he had co-operated with the South African police investigation.
The South African authorities have appealed against the decision to grant conditional bail. The appeal will be heard on Friday and Dewani will remain in custody until then.
http://www.news24.com/SouthAfrica/News/Unforgivable-if-Shrien-involved-Annis-sister-20101209