The sister of the Dutch woman allegedly killed by her ex-husband in a Queens triple-murder-suicide realized her sibling was dead — during an internet search.

The sister was in the Netherlands when a worried friend of her sibling Linda Olthof “reached out to’’ her and said Olthof wasn’t able to be reached Monday night, said NYPD Chief of Detectives Dermot Shea on Tuesday. The pal had been visiting New York City with Olthof and her 6-year-old son.

Olthof’s sibling then “Googled New York’’ and saw news reports about the Queens slayings, which hadn’t yet identified the victims by name, Shea said.

“When [the sibling] saw those stories, she realized something had befallen her sister,’’ the police official said.