Gimeno and the girls went missing on April 27 in Tenerife, the largest island in the archipelago off West Africa.
Their mother, who is divorced from the girls' father, alleged he told her she would never see them again.
Investigators launched a wide search on land and sea but narrowed in on the waters off Tenerife after Gimeno's boat was found empty and drifting at sea.
Interpol also joined the search, publishing the photographs of the two girls and issuing so-called “yellow notices" aimed at locating missing persons.
The girls’ mother had posted several videos of her daughters online in an effort to help find them.
Speaking at a women’s forum on Friday, Queen Letizia expressed “pain and sadness” for the deaths of Olivia and a 17-year-old woman killed by her partner in southern Spain earlier this week.
“I don’t think there is anyone this morning who is not trying to put themselves in the shoes of the people who love these murdered girls,” Letizia said.
Women’s rights organizations have called for protests later on Friday across Spain against the recent uptick of violence against women.
Men have killed at least 18 women so far this year in Spain, according to data from the Equality Ministry, which has recorded 1,096 deaths since 2003, when the country started keeping data on crimes of gender violence. Abusers often use children as means to inflict harm on their partners or ex-partners: since 2013, 39 minors have been murdered in the country by their biological fathers.