There may be legal considerations. Depending on where you live and your role in the victim's life, you may be required to report her situation to the authorities. 

Teachers, ministers, counselors and other professionals are often "mandated reporters," or people who are required by law to report child abuse, including incest. Check RAINN's mandatory reporting database if you are not sure whether you are required to make a report. 

Even if you are not a mandated reporter, calling Child Protective Services (CPS) may be the best way you can help protect the victim. CPS workers will be able to investigate the situation in greater detail and take steps to protect the victim. You can make reports to CPS anonymously if you are concerned about the victim's family knowing who made the report. Find the number for CPS in RAINN's mandatory reporting database. Information is listed by state. 

You may also contact the local police department, particularly if you have concerns about the victim's safety at the time you find out about the incest.