Is familial sexual abuse a big problem in Japan?
I would say it is a very serious problem.
The total victmization rate of underage is 16.5%, if we only consider underage victim suffered intercourse sexual abuse, the number would be 2.2%. 12% of the crime were commited by relatives, so it's about 0.264% of all underages suffered from sexual penetration by family members.
If you consider excessive body touch that includes unvoluntary body touching, kissing, touching the genitalia and body fluids by relatives, the number would rise to an astonishing 1%.
That means there are at least 150,000 children has suffered from some type of physical familial sexual abuse in Japan.
Full report by Japanese Government:
Also please note this report didn't include numbers of victims under 16 years old.