My brother in law is a child molester, Suggestions?

My first inclination was to kill him, that would cause more family hardship and is apparently illegal. He has been molesting my niece (not his daughter, he claims in justification) since she was seven, she is fifteen now. She's a bundle of damaged goods. His wife asked me to keep quiet, not kill him while she works this treacherous betrayal out in her heart. There are younger children in the household (unmolested, his children) and would appreciate any input before I do the right thing.