Authorities said they think the missing girls, Alexandria, 12, and Kyliyah, 8, are still with Adam Mayes, nearly two weeks after he fled with them.
In a Wednesday interview with The Associated Press, Mayes' mother-in-law, Josie Tate, said he thought the missing sisters might actually have been his daughters and that it caused problems in his marriage to her daughter, Teresa Mayes, who is jailed in the case.
"She was tired of him doting on those two little girls that he claimed were his," Tate said.
Authorities have put Mayes on the FBI's Ten Most Wanted List and urged him to surrender.
"Turn the girls in, and then peaceably and safely turn yourself in to law enforcement," FBI Special Agent Aaron Ford said at a Wednesday news conference. "We believe Mayes could be anywhere in the United States, and we are extremely concerned for the safety of the girls."
Mayes and his wife, Teresa, were charged Wednesday with first-degree murder in the deaths of Jo Ann Bain, 31, and her daughter, Adrienne, 14. Their bodies were found buried outside the Mayes' home near Guntown, Miss., a week after they were reported missing by Jo Ann Bain's husband, Gary.
The reward for information leading to Mayes' arrest is now at more than $100,000.
Mayes' wife told investigators he killed Jo Ann and Adrienne Bain at their Whiteville, Tenn., home on April 27 so he could abduct the two young sisters who remain missing, according to court documents filed Wednesday.
Authorities refused to comment on the motive for the April 27 slayings and abductions at the news conference.
Teresa Mayes told investigators that after she saw her husband kill the two in the garage at the Bain home near Whiteville, Tenn., she drove him, the younger girls and the bodies to Mississippi, according to affidavits filed in court.
Since the manhunt began for Mayes, people who knew him and the Bains have described him as unusually close to the family and the girls. He was described as a friend of Gary Bain, and the children considered him an uncle.
In an earlier interview, Tate's daughter, Bobbi Booth, said Teresa Mayes suspected her husband was having an affair with Jo Ann Bain.
Mayes was often at the Bain home. Authorities said he was spending the night there before the mother and daughters were reported missing so he could help the family to pack for a planned move to Tucson, Ariz., and then drive their belongings west.
A Facebook page Adam Mayes was using under an alias that was confirmed by law enforcement showed several photos of him and the Bain girls. One picture depicts Mayes and two of the girls smiling, all leaning next to one another cheek to cheek. The page has since been taken down.
Authorities said Alexandria has brown hair and hazel eyes and is 5 feet tall and 105 pounds. Kyliyah has blonde hair and brown eyes and is 4 feet tall and 57 pounds.
Mayes' mother-in-law, who lives in Chatsworth, Ga., said she's known him for 25 years but didn't approve of him because his family never seemed to stay in one place and he couldn't hold down a job.
"Teresa's father and I begged her: `Do not marry him, do not go off with him, do not live with him,"' she said.
Tate described her daughter as a slow learner who spent her school life in special education. Teresa Mayes was also incapable of having her own children, she said.
The mother said she believed Mayes had threatened her daughter and perhaps his own mother, Mary Frances Mayes, who has also been charged with conspiracy to commit especially aggravated kidnapping.
Mary Mayes' attorney, Somerville attorney Terry Dycus, said his client maintains she is not guilty. Dycus said it was too early to discuss what the mother's defense would be.
"She agrees with the authorities that he's possibly dangerous, but her main concern is that the children be returned immediately and safely," Dycus said.
Teresa Mayes faces six felony counts in the case: two first-degree murder charges and four especially aggravated kidnapping charges.
"The feelings I have for Adam are as close to hate as I'll ever come because he's destroyed not only the Bain family but he's destroyed my family too," Tate said.