@fossyant you have my sympathies. Families are very complicated things and our issues have flared up again with the sale of my late grandfather's home.
There is probably not much more you can do but stand by your immediate family and support them through this difficult period.
Has the camera been installed? If it can prove things one way or another it may help even if the nephew ends up with a record. If it is him, he may deserve it. I know 20 years ago I should have made sure a certain member if our family ended up with a record. I was able to protect who I needed protecting, we did it 'in house' but it would have been better in the long run if it had been official despite the problems it would have caused at the time.
Best wishes and I hope you get it sorted.
My grannie squirreled away over £5,000 in various places around the home during the years she was affected by her first couple of strokes. Most of it was not doing until many years later simply because of where she had hidden it. It had nothing to do with insecurity. She was trying to put away some money to leave to her grandchildren after her death because she was only given an allowance by my grandfather for groceries and the likes and hiding a £10 here and there each week was her way of dealing with never having had her own bank account. Her intentions only came to light a few weeks ago when I found an unopened letter in amongst her personal affects which I had been given to sort through some 5 or more years after her death and only after the death of her husband as well. He never touched her belongings after her death, just closed the door and... My grandmother's intentions are being honoured by my mother out of her party of her inheritance. The other party refuses to acknowledge them then at because it would mean less for him to inherit and well even my grandfather's bequeathment (? Correct word) is coming from my mother's inheritance and not before the division...
Different situation I know but families are so very difficult. I hope it sorts itself out and quickly. One way out another it will deal with the stress and at least your family will know who they can and can't trust again.