Sarah Nicole Henderson, 29, of Mabank, has been charged with two counts of capital murder after fatally shooting Kenlie, five, and KayLee Danielle, seven.
Her husband, Jacob, who is stepfather to the girls, told deputies that she told him: 'Babe, I just shot the kids' before he found them dead in the living room.
Earlier, around 11.30pm on Wednesday, Jacob called 911 claiming that his wife was suicidal.
When Henderson County deputies got to the scene, the couple said everything everything was fine.
'No one was in any danger. No one was suicidal, no signs of violence, no signs of anything,' Henderson County Sheriff Botie Hillhouse told WFAA.
Three hours later, early Thursday, Jacob called 911 again - this time to report the girls' murders.
'The only thing she told us was that she had smoked some marijuana early in the evening. She was very blunt. We just don't have any answers as to why,' said Sheriff Hillhouse.
Her bond was set Friday at $2million, and she was being held at the Henderson County Jail on suicide watch.
Hillhouse said Sarah confessed when talking to investigators and said she had been planning to kill the girls and her husband for several weeks.
She tried to kill her husband too, but the gun malfunctioned, authorities said.
Jacob had initially called 911 about 11.30pm on Wednesday to ask for medical help for his wife. According to a recording of that call provided by the sheriff's office, he tells operators that she won't listen to him and that she is 'freaking out like someone is out to get her'.
He calls back to cancel the call a few minutes later saying that Sarah told him she's fine now and is acting normally. The call was coded as a potential suicide when reported to the sheriff's office. Deputies responded to the house about 11.45pm, when the couple told deputies they were OK and didn't need help.
Jacob called 911 again about 2.24am on Thursday to report that his wife had shot the girls. He can be heard sobbing and asking why she had done it in the recording released by the sheriff's office.
'She was asleep when I went to sleep last night. I woke up and she came in there said: "Babe, I just shot the kids". And I didn't want to believe it. I went in there and they were dead,' Jacob tells the 911 dispatcher.
He said the girls were asleep in the living room, and that they had both been shot in the head.
Sarah can be heard in the background asking her husband to kill her or shoot her, saying people are after her and that they're coming. At one point in the recording she yells: 'God forgive me' and 'What did I do, Lord? What did I do?'
Hillhouse said the girls' biological father has been notified. He added that Sarah has an older son who lives with his father and who was not in the house at the time of the shooting.
In 2010, Child Protective Services was called to investigate Sarah, but the information is confidential.
A GoFundMe account has been set up to help cover the girls' funeral expenses. So far, $5,530 has been raised out of a $7,000 goal.