"We're very worried about them," Carla Burgos said.
The sisters get scheduled visitation with the children, which they cherish.
"It's very difficult. It's very difficult for them, it's very difficult for us," Jeane Burgos said. "At the time we are together, we try to make a happy and fun time for them."
Beresford-Redman was found dead in a sewer April 8 at the post Cancun resort where she was staying with her husband and children.
Shortly before Mexican authorities charged her husband with murder, ABC News confirmed he took out two life insurance policies on his wife a week before her body was found.
Mexican officials cited "asphyxiation by suffocation" as the cause of her death.
Sources told ABC News that one of those life insurance policies paid out $50,000 in case of accidental death while traveling. The other would pay $500,000.
The policies were also taken out on the couple's children. The policies were purchased in late March; one from Worldwide Insurance, the other from InsureMyTrip.com.