He met Mr Leo Chavez only once before the latter married his younger sister, Ms Tara Irdawati Isnin, in August.
And somehow, Mr Irwan Isnin did not have a very good impression of his brother-in-law.
"I didn't like him at all," Mr Irwan told The New Paper in a short e-mail interview on Thursday.
He declined to say why.
And before Ms Tara, 33, a Singaporean, left for Hawaii to marry the American man, Mr Irwan told her to take care and to call their mother.
Ms Tara, who was the corporate communications manager of real estate company Savills, was found dead with multiple gunshot wounds to her head in her 19th-floor apartment at the Ala Wai Townhouse in Waikiki.
Honolulu police were alerted to the shooting at 4.30am on Monday (10.30pm Singapore time).
Their public affairs spokesman Teresa Bell told The Straits Times Online that a 40-year-old man was found with "apparent self-inflicted injuries" in the same apartment where Ms Tara's body was discovered.
He is hospitalised and no arrest has been made yet, Ms Bell added.
The man is understood to be Mr Chavez, who reportedly serves in the US Army.