(CNN) After one woman discovered eight unknown siblings following a commercial DNA test, she investigated and learned that her biological father was her mother's fertility doctor, according to court documents from Marion County, Indiana. Dr. Donald Cline had told his patients he was inseminating them with "fresh sperm" from a medical student or resident.

Now, Cline has entered a plea of not guilty to two charges of obstruction of justice.

"It was unethical, what he did. He was telling his patients one thing and doing another," the woman, who wishes to remain anonymous, told CNN affiliate WXIN.

"This kind of thing happens very rarely, and the American Society for Reproductive Medicine doesn't keep numbers on it," said Eleanor Nicoll, a spokeswoman for the society.

Finding half-sisters

About 6% of married women in the United States are unable to get pregnant after a year of unprotected sex, while 12% of women have difficulty getting pregnant or carrying a child to term, according to the Centers for Disease Control and Prevention (PDF). Between 2006 and 2010, the CDC also found that about 12% of women (7.3 million) or their husbands or partners had used infertility services at some point in their lives.

The Cline case began with one woman and quickly led to her half-sisters.

Jacoba Ballard's investigation into her identity began when she became curious about her history, the court documents explain.

Similarly, Kristy Killion became interested in her roots when she reached adulthood. She knew that her parents had used a fertility clinic, so she called Cline's office directly. The doctor told her that although he recognized her parents' names, he had shredded the medical records, and so he could not help her find her sperm donor. Accepting what he told her, she listed herself on a website that connects donor children.

After Killion was matched to Ballard, the two half-sisters talked, and then both took a DNA test. At this point, they found more siblings -- nine people total -- and then became suspicious "because there were only supposed to be three successful pregnancies using the same sperm donor," the court documents state.

Face-to-face meeting

Eventually, they discovered a genetic link to Doug Cline, the doctor's son. When contacted on Facebook, Doug told them his father had admitted donating sperm samples to different laboratories over the years. Surprised, confused and wanting to know more, Ballard arranged to speak with Doug's father: the doctor himself.

During a face-to-face meeting with Ballard, Killion and four other siblings, Cline confessed he'd donated his sperm, not through a bank, about 50 times and "admitted to doing wrong by inseminating the women with his own semen, but felt that he was helping women because they really wanted a baby," the court documents say. He also said he felt pressured into doing what he'd done because he didn't always have access to fresh sperm.

In 2014, Killion and Ballard sent complaints to the Indiana attorney general's office, Marion County Deputy Prosecutor Tim DeLaney told WXIN. During a subsequent legal investigation, Cline allegedly lied about his medical practices in a letter to the office; the falsehood became obvious when "the result from the DNA analysis test factually state that Dr. Donald Cline is the biological father of Kristy Killion and Jacoba Ballard," the court documents state.

Cline retired from his medical practice at Reproductive Endocrinology Associates in 2009. Now, he faces up to five years in prison if convicted of both counts of obstruction of justice, said Peg McLeish, a spokeswoman for the Marion County Prosecutor's Office. Cline's next court hearing is a pretrial conference October 17.

Tracy Betz, Cline's lawyer, did not respond to a request for a comment on the case.

"There's clearly an ethical concern or breach," said Steve Boreman, a California-based lawyer who teaches professional ethics to physicians, physician's assistants, nurses and pharmacists who have "stepped over the line" and so must take his class under court order.

Ethics aside, the possible legal ramifications are unclear. Boreman, who is not involved in this case, cannot comment on an unseen contract between Cline and his patients, but he noted that any patient undergoing a medical procedure would sign off on informed consent, and in this case, patients clearly were not "fully informed."

Asked whether Cline's many children might sue for back payments of child support, Boreman said he does not know.

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Typically, when people undergo fertility treatments, they sign legal forms that "hold harmless" both the donor and the clinic for support claims and other issues, he explained. However, a contract can be voided when there's an intervening criminal act, he said. Considering that Cline "breached his overall duty to be honest and upright," there might be an argument to be made that "his criminal or quasicriminal act of deceit" would void any shield of liability for child support, Boreman said.

Though paternity lawsuits may be unclear, legal matters might someday touch the new siblings.

"The current siblings don't have an obligation to one another -- except if there is a death and no will, things could get messy," Boreman said.