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.
The case has been classified as murder.
Ms Tara's close friend, Ms Sharani Khamis, 32, told TNP that she met Mr Chavez twice.
Like Mr Irwan, she also did not have a good impression of him.
Ms Tara was previously married to an Australian man in 2006. They have a daughter who is now four years old.
Shin Min Daily News reported on Thursday that Ms Tara's marriage to Mr Chavez was allegedly not a happy one.
However, Ms Sharani, who was authorised by the family to speak to TNP, said: "There are a lot of inaccuracies about claims of divorce and the fights between the couple.
"We can start with leaving her family in peace. They just lost a loved one in such a shocking manner - let's respect that.
"The law will handle the rest."
Mr Irwan said that he was close to his sister until their teenage years, when she moved to live with their grandmother.
"As young kids, both of us were playful," he recalled.
"But which siblings don't get into fights and such? Tara was always very naughty and so was I.
"I will miss everything about her as she was my only sister."
Ms Sharani described her friend as an intelligent and independent woman who was well-loved by her family and friends.
She said: "We used to meet up every fortnight for dinner and such. I broke down when I found out that she's gone."
Ms Tara will also be missed at one of her favourite hangouts, Cuba Libre bar at Clarke Quay.
Its manager Anil Rahim, 31, said: "She was very bubbly and was always the life of the party. I was shocked when I found out about her death."
Mr Irwan said that arrangements have been made for his sister's body to be brought back to Singapore.
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