Paul, "It's either a goldmine or a minefield" certainly says it perfectly. I hesitated because, as you point out, the purpose of the article is to show the prevalence of incest in Japan, but the paragraph described so exactly what you are facing that I decided to risk it.
Clearly, one doesn't need to look any farther than culture to explain her wanting to co-sleep with the children, but IF something did happen to her while sleeping in the family bed when she was a child, it might explain her reluctance to talk about it.
I hesitated because, as you point out, the purpose of the article is to show the prevalence of incest in Japan, but the paragraph described so exactly what you are facing that I decided to risk it. 
