Plainfield Police Lt. Kevin O'Brien said the girls are in Robert Wood Johnson University Hospital and are "responsive," The (Newark, N.J.) Star-Ledger reported Friday. They were dehydrated when they were discovered.
Police found the twins Wednesday while doing a welfare check on Alice Jackson. Her family had become concerned because they had not heard from her for a week.
O'Brien said Jackson apparently died of natural causes. He said nobody had seen or spoken to her since March 29.
George Jones, the girls' father, who lives in Baltimore, said Jackson had been sick, although he did not go into details.
"She just went to sleep and didn't wake up," he said.
In a similar case last week, a 4-year-old boy was found in Union with the body of his mother, Kiana Workman. She is also believed to have died of natural causes.