Some professionals' writing on sexual activity among siblings and cousins.

Not saying these are the most important studies, but that they give some definitions and understanding.

The psychpage link has a bibliography that might help a search for other information. A look at these papers suggests that the behavior of Josh Duggar well fits known prevalence of features of this behavior. One feature that I found notatble is that this behavior happens often enough that most people who have 5 or more associates are likely to know either a perpetrator or a victim.

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Sibling Sexual Abuse
http://www.psychpage.com/family/library/sib_abuse2.htm

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Incidence of sibling or cousin sexual abuse varies greatly among studies, ranging from 10% to 40% among those reporting sexual abuse, although sibling and cousin sexual abuse are about equal in such samples (De Jong, 1988).
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Causes

De Jong (1988) and Daie et al (1989) present several factors in families that can lead to sibling or cousin abuse. Abuse can arise in an environment that:

--fails to protect the child, though poor supervision/monitoring and poor choices regarding babysitters and surrogate caretakers; examples include allowing children to remain in the care known siblings abusers, or a general lack of supervision of the children and knowledge about their activities in and out of the home

--fails to set appropriate boundaries though inappropriate sleeping arrangements and clear parent-child hierarchies for child care; examples include allowing older opposite-sex children to sleep with parents in the absence of the same-sex parent, or flagrant nudity or lack of privacy in the family. Inappropriate parental interests in children's sexual development and experiences, or extremely relaxed views of sex, can both lead to greater freedom to explore sexual relations with peers and siblings. Others (Smith and Israel, 1987) argue that open parental sexual activity, especially in cases where one parent is having an affair, is an especially significant indicator; parents turn outside the family for their needs, marking a disintegration of family structure and an increase in anxiety in siblings. This may promote a sexual relationship between siblings in an attempt to nurture each other

--lacks supervision and care by two parents; the absence of a parent can arise through physical absence from the home (e.g., divorce, death, abandonment, or work), or emotional absence due to physical or psychological illness (e.g., hospitalization due to depression, alcoholism, or dependent personality disorder). Some (Smith and Israel, 1987) have argued that this dynamic is one of the most salient of sibling incest families

-- is based on a multi-familial home were older children have caretaking responsibility for younger children; combined with poor boundaries, this situation can allow older children to pressure younger children into meeting their sexual interests and needs allows physical abuse and neglect; Daie cites a case in which a physically abused boy in turn abused his sister, with sexual abuse being a minor part at first, but becoming the predominant form of the abuse over time has a rigid family structure, discouraging open communication between children and parents, children and outsiders, and even between parents. They often show devaluing attitudes toward women, and enforce parental authority (often the father's power) unquestioningly over the family. This often leads to isolation, secretiveness, and enmeshment.


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Sex among siblings: A survey on prevalence, variety, and effects
Archives of Sexual Behavior
June 1980, Volume 9, Issue 3, pp 171-194

Abstract

In a survey of 796 undergraduates at six New England colleges and universities, 15% of the females and 10% of the males reported some type of sexual experience involving a sibling. Fondling and touching of the genitals were the most common activities in all age categories. One-fourth of the experiences could be described as exploitative either because force was used or because there was a large age disparity between the partners. Reactions to the experiences were equally divided among those who considered them positive and those who considered them negative. Females were more likely than males to have been exploited and feel badly about it. Few participants of either sex ever told anyone. The research finds evidence that such experience may have long-term effects on sexual development. Females who report sibling sexual experiences, both positive and negative, have substantially higher levels of current sexual activity. Their level of sexual self-esteem may also have been affected, but more selectively. Those with positive sibling experiences after age 9 have more sexual self-esteem. However, experiences with much older siblings taking place before age 9 are associated with generally lower levels of self-esteem and no increase in current sexual activity.


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"Sexual interactions among siblings and cousins: experimentation or exploitation?"
Dejong
Child Abuse Negl. 1989;13(2):271-9.

Abstract

Incest by cousins has not been well documented compared with sibling incest. The purpose of this study was to describe the features of incest by cousins and siblings presenting to a sexual assault center and to differentiate cases of abusive behavior from normal sexual exploration. Four criteria were considered indicative of abusive behavior: (1) age difference of greater than or equal to 5 years between victim and perpetrator; (2) use of force, threat, or authority by abuser; (3) attempted penile penetration; and (4) documented injury in victim. Of 831 sexually abused children less than 14 years of age evaluated for sexual assault complaints, 49 cases of cousin incest (5.9%) and 35 cases of sibling incest (4.2%) were identified. A total of 54 male cousins abused 8 boys and 41 girls; brothers abused 3 boys and 32 girls. The victims' median age was 5 years for cousins and 7 years for siblings. The perpetrators' mean age was 16.2 years for cousins and 15.5 years for siblings, with only 16 (19%) of all perpetrators being greater than 16 years old. Of the perpetrators, 66 (79%) were greater than or equal to 5 years older than their victims. All 18 victims with age differences of less than 5 years met one or more of the other abusive criteria. A similar pattern of adolescent perpetrators having abusive sexual contact with young children was demonstrated by analysis of cousin incest and sibling incest in this study. Appropriate case management requires understanding of normal sexual experimentation and recognition of the abusive nature of these cases.