Durham, N.C. — The sister of a woman who authorities believe kidnapped 17-month-old twins said Wednesday the case is a complicated and emotional one that perhaps was building over time.

Durham police and the FBI are investigating the disappearance of Tyler and Holly Needham, who were taken from their adoptive parents on Dec. 22 for a scheduled monthly visitation by their birth mother, Allison Quets.

Police said Tuesday, however, that Quets, 49, never returned Sunday evening and that she and the children might be in Kentucky or Florida.

"God forbid that a loving mother should face criminal charges for wanting to be with her children," said Quets' sister, Gail Quets.

After her sister got pregnant through in vitro fertilization, Gail Quets said, the pregnancy was difficult and her sister suffered from extreme nausea and had to be fed through a tube.

"She weighed less after nine months than she did before she was pregnant," Gail Quets said, adding that her sister was so weak that she worried she would not be able to care for the twins.

"She was absolutely pressured into giving them up for adoption," Gail Quets said.

Allison Quets found Kevin and Denise Needham through a friend, but regretted her decision to give up her children 12 hours after she signed adoption papers, her sister said.

"She's been fighting since Day 1," Gail Quets said. "She has spent more than $400,000—all of her life savings—on the legal process."

The custody battle is playing out in Jacksonville, Fla., where Allison Quets lives. Her sister said she has been renting an apartment in Durham so that she can have monthly visits with the children, who now live in Apex.

A neighbor said she saw a stroller outside Quets' apartment on Christmas Eve, but has not seen her since.

Investigators think Quets might be traveling in a white Plymouth van with North Carolina license LRJ-6644 or Florida license E377HZ.

Anyone with information about the whereabouts of the three is asked to call 911 or the Durham Police Department at 919-560-4427.

Durham police would not comment on the case Wednesday and directed all questions to the FBI, which is now investigating.

WRAL's calls to the FBI were not immediately returned Wednesday, so it was unclear exactly why an Amber Alert has not been issued in the case. Typically, one is only issued if authorities believe an abducted child is in danger.

The children's adoptive family said Wednesday that investigators advised them not to comment.

Gail Quets said she does not know where her sister is, but that she was not worried about the children.

"I'm not that worried, because I know my sister is a very smart and capable person," she said. "And I know she loves her children very much and would never endanger them."