When I was in college one of our classes was on sexuality. The statistics at that time were that 60% of ALL "reported" cases of incest was between siblings. That right, of the "reported" cases it was 60%, the other 40 was father, uncle, aunt, mother, cousin..... I just wonder how many times it went unreported.
It is because they are left unsupervised, they see each other naked all the time, they get exposed to the other person and it becomes familiar. They are convenient and can offer a temptation they are ill equipped to handle.
My daughter's step brothers would wrestle her to the ground and strip her clothes off and push her outside every day after school then stand inside laughing. It was very hard on her and she didn't tell anyone until she was an adult.
Incest happens all the time. It's not something that is new.
My friend took in her hubby's half sister. She is developmentally disabled as are 4 out of 6 of the kids in this family, they only have low IQ's that is all, no other qualifiers exist, they only have below 70 IQ's. My friends husband is very smart and the youngest son is perfectly normal too, he is actually very smart and makes a lot of money.
They got the girl because they found out she was pregnant, they tried to stop the abortion dad had gotten approved and were only able to get a court order for blood work from the fetus. The dad got her an abortion before the baby could be born and the paternity testing done. The court ordered an DNA test on the baby/fetus after the abortion and the baby's fathers DNA was so close to her own that there was no question the baby was either the girls fathers or one of the brothers that had developmental disabilities. The doc did say the baby was perfectly normal, it had no genetic tags that would lead him to believe the baby would be disabled.
The girl had been living with mom, come to fine out mom would tape her to the bed with duct tape and sell her for beer and cigarettes. She would laugh and watch too. When the dad found out he took her and (his story in court) then he would leave the kids at home when he went to work, they were teenagers.
He always said the baby was not his and it must have been one of the brothers. He stated in court he had caught them going at it and stopped it. The girl was so traumatized by all this that she can't even talk to a male, who could blame her.
But my point is, the sex drive is a strong deeply ingrained part of our human nature. We live to have sex to have children to live.....all human beings have a sex drive. It is our human nature. Kids are there, living it and when unsupervised they will experiment and that can lead to further inappropriate contact that will eventually lead to sexual intercourse.
I would make sure this got called in. If you know any information about this family you have an obligation to call it in yourself and tell them all that you know. It is something you have to decide, I could not knowingly let this go on if there was any way I could help end this for that child.
I would call the state office of child welfare and tell them exactly what you have told us, the name of the child if you know it, the school she goes to, how old she is, what she looks like, etc...each little bit of information might eventually lead them to the family.