A few months after her daughter was born, Miriam Carey was diagnosed with postpartum depression with psychosis, her sister told CNN's AC360.
Postpartum psychosis can cause delusions and paranoia, according to medical experts.
"There wasn't a pattern. It was something that occurred suddenly," Amy Carey-Jones said. "She seemed overwhelmed. There was a lot of stress.
"There was not moments of her walking around with delusions. That was not what was going on."
But her sister was making progress with the help of counseling and medications.
Carey-Jones said her sister recently told her that the doctors told her she didn't need the medication anymore. "They tapered her off the medications, and she said she felt fine," Carey-Jones said.
She declined to discuss what medication Miriam Carey had taken.