Dallas - As a van full of family members tried to escape Harvey, violent floodwaters engulfed the vehicle and six people are presumed dead, including four siblings aged 6 to 16, a relative said.Virginia Saldivar said on Monday that when her in-laws' northeast Houston home began to flood early on Sunday, her brother-in-law Samuel Saldivar borrowed her husband's van and drove to pick up the relatives.She said at some point on their way to safety, a strong current lifted the van and pitched it forward into Greens Bayou.Samuel Saldivar climbed out of the driver-side window but the van's sliding door was partially submerged and would not open, Virginia Saldivar said.He yelled at the children to try to escape out the back, but they were unable. Virginia Saldivar said her brother-in-law could only watch as the van disappeared under water."Sam calls my husband and tells him: 'They're gone,'" Saldivar said. "That's when my husband dropped the phone and started screaming."READ: Houston shelters overwhelmed as Louisiana braces for monster stormFloodingVirginia Saldivar believes her husband's parents, 84-year-old Manuel Saldivar and Belia Saldivar, 81, drowned along with their grandchildren Daisy, Xavier, Dominic and Devy.Virginia Saldivar said she lives in the same neighbourhood as her relatives, but she and her husband left during a calm spell on Saturday to watch the Mayweather-McGregor boxing match.The children's mother had left the four at home, she said. The widespread flooding prevented them from getting home until Sunday afternoon.She said the Coast Guard told her family they couldn't search for the bodies until the water recedes. Saldivar said she has not yet told the children's father, her son, who she said is in prison for violating parole.A spokesperson for Houston's Office for Emergency Management was unable to confirm the presumed deaths, first reported by KHOU-TV.Harris County Sheriff's Office spokesperson Jason Spencer said Samuel Saldivar told them he was driving eastbound on Sunday when rising water in Greens Bayou overcame the van, KHOU reported.According to Spencer, Saldivar said the victims were his elderly parents and four great-nieces and nephews.Rescue boats fill a flooded street as flood victims are evacuated as floodwaters from Tropical Storm Harvey rise in Houston. (David J Phillip, AP)Virginia and her husband fled their home on Monday evening when water rose to about 2.4m outside their front door. Volunteers helped get the couple to dry land.As soon as it is safe to return to Houston, Saldivar said from a relative's home in Humble, Texas, she and her family will go looking for the bodies themselves."Finding my babies - that's my ultimate goal," she said.