Sibling Molestation

My 3 yr old little girl told me that her 9yr old brother licked her vagina....she also told me that he told her if she licked his he would stop, she said she did, but he didnt stop......I have spoke with me son about this....told him how wrong it was, that he will not be allowed to treat his sister that way....I think he might have done this to another boy cousin.....but not sure. What should I do? Is this curiosity or something bigger? He was caught with very graphic p 0 r no pics on his computer account last year and has not been allowed on the internet since.....could he have got this from that? Please, any advice will be appreciated.