have been researching this one a lot my self, try googling child on child sexual abuse and also child sexual development
also i had a lot of help from my therapist on providing insight on the issue of children abusing other children. its true children do learn it from somewhere, there could be many children linked in the chain to your husbands abuse or there may only be a few.
I find it hard to call it abuse, because to me abuse implies intent, abuse of power, trust etc. i was approx 5 when it happened to me by a little girl who was about the same age as me.
later down the track her sister also showed signs of sexual knowledge beyond her years with my sister. while i am angry that my sexual development was impaired, i dont think that at the age of around 5 my friend had any concept of what she was doing. its true abuse such as this does feel physically good, she probably thought she was doing something nice for me. problem is that my sexual development was sped up while my emotional development was left behind. i did not have the capacity to understand what was happening i really wouldnt even have probably been articulate what was happening to me to my parents anyway because i hadnt developed a vocab at that age that would have told them anything was outside of the normal course of sexual development in children.
wikipedia has some information and a lot of references at the bottom that provide more resources, and there is a dissertation somewhere on the internet that provides some insight also. A qualified psychologist experienced in either csa or child sexual development will be a tremendous resource also not just for your husband and his brother but for you also.
hope this helps, im no expert im still learning myself so if you find anything you think may help me please post so i can learn to.