So sorry to hear this. I knew this would happen which is why I sent you a pm in order to try and help you. I have been through this 3.5 years, and I have helped other mothers in the UK, USA, Canada, France, Belgium, in fact the whole of Western Europe, Australia, New Caledonia (French colony in the Pacific) etc - there seems to be a template in all these places - the investigations are a joke. It is not a mistake. That is the usual template. They ignore/minise evidence or waste time until the evidence has been gotten rid of by the named perp, and then they turn around and tell the victim and the family there is 'insufficient evidence' to proceed to a prosecution. This is standard. If you actually find a Police force that did a thorough investigation, it is extremely rare, and when they happen they are usually with regards to non-familial paedophiles. The reasons are too long for me to go into here. In any case, it does not help. What you need is practical help to ensure the investigation succeeds. And your sister and her kids are safe. You need to know and warn your sister with respect to her daughter (the niece you mentioned), if she is this man's biological daughter, or named as a child of the family even if she is not his biological daughter subject to your jurisdiction - you need to start making safety plans now. Usually if the investigation does not lead to a prosecution and conviction, he will take your sister to family court for custody of or access to your niece if she is his daughter or a child of the family - and that is where things will start to get really really bad. This stage you are in now, is the beginning, and the easy stage. So it is crucial you make sure they get it right at this stage because if they don't, it will get worse. I am sorry, I wish there was a better way to say this, but there isn't. I am living through the hell right now. And I do not wish this on my worst enemy. I will be more than happy to point you in the right direction so that you and your sister avoid the mistakes I made in trusting the authorities to do their jobs and protect my children. You do not need the agony that will ensue if he is not prosecuted.
You are at a stage now, where you still have some level of control. In the family court, abusive men rule. Evidence is ignored and the abused are forced back into the hands of their abusers. Have you ever heard of Parental Alienation Syndrome by Richard Gardner? That is the familial paedophile's favoured defence in family courts all across the western hemisphere. Google all about it and get yourself armed.
If you need help and pointers as to what to do whilst the investigation is still early, I suggest you respond to my pm so that I can guide you in as much way as possible. If you choose not to, I will respect your decision.
I wish you and your family the best of luck.
A Loving Mum.
Daily I worry for the safety of my young sons - but worry achieves nothing! So I pray for their safety!