In a statement, Rajesh said Sunanda was one of the strongest personalities and to even remotely think that she would do any bodily harm to herself is "preposterous" and "should not be believed".
Sunanda's brother said it was unimaginable to believe that Shashi could harm her
He also sought speedy investigation into the case "to put an end to the rumours that are hurting" the family. The probe into the high-profile case was transferred to the Crime Branch on Thursday considering its "sensitive nature" and "complexities" involved.
"I have been with both Shashi and my sister throughout their journey together and can say with utmost confidence that they were a couple madly in love with each other. Differences, if any, were momentary and do happen in every household, and we do not believe that they have any bearing on the incident," he said.
Rejecting speculation in the media about Sunanda's death, he said both he and his father were "deeply disturbed" at the "slanderous statements" that have been made in the media regarding her death.
"I have known my sister to be an angel at heart but to often impulsively say things she did not mean and withdraw them later."
Sunanda, 52, was found dead in a 5-star hotel in south Delhi on January 17 night, a day after her Twitter spat with Pakistani journalist Mehr Tarar over an alleged affair with Tharoor.
"My sister was one of the strongest personalities and to even remotely think that she would do any bodily harm to herself is preposterous and should not be believed. Also to think that my brother-in-law would do any harm to her is unimaginable," he said.
He requested that the matter be put to rest so that peace can be bestowed on the departed soul.