Sexual assault of children often includes incest.

Incest is sexual contact between persons who are so closely related that their marriage is illegal (e.g., parents and children, uncles/aunts and nieces/nephews, etc.). This usually takes the form of an older family member sexually abusing a child or adolescent.
There are very few reliable statistics about how often incest occurs. It’s difficult to know how many people are affected by incest because many incest situations never get reported. There are many reasons that the victim might not report the abuse.

The victim may be told that what is happening is normal or happens in every family, and doesn’t realize that it is a form of abuse. The youngster may not know that help is available or who they can talk to. Children may be afraid of what will happen if they tell someone, and may also be concerned about how many people will react when they hear about the abuse.

Incest is especially damaging because it disrupts the child’s primary support system, the family.

When the abuser is someone in the family, the family may not be able to provide support or a sense of safety. Since the children (especially younger children) often have limited resources outside the family, it can be very hard for them to recover from incest.

Incest can damage a child’s ability to trust, since the people who were supposed to protect and care for them have abused them. Survivors of incest sometimes have difficulty developing trusting relationships

It can also be very damaging for a child if a non-abusing parent is aware of the abuse and chooses—for whatever reason—not to take action to stop it.

Please see: Rape, Abuse & Incest National Network
RAINN: Child Sexual Abuse and Incest