I have worked with sex offenders for many years. All I can say is that befriending a single woman with children is a classic behaviour trait. You sister may be right and he has realised he was an idiot in the past, but she could also be wrong. shelscrape is absolutely right on this Op, the bottom line is regardless of whether he is a nice guy or not, your sister is gambling with her childrens safety. Where is their father in all this? He has a right to know who his children are around as well.
Todayisnotthedsay is right, please be careful who you trust. Go with your gut instincts, as you already said the Policewoman you spoke to was concerned about the situation, that should at the very least indicate you must continue to take action.
A suspended sentence is not to be taken lightly at all! It is still a formal criminal prosecution. Accidentally accessing images are dealt with by reporting them so they can be traced, especially if it has been done on a work computer. This is a very common excuse used by many convicted paedophiles. Also the majority of work intranet systems have firewall protection software to prevent porn sites from placing cookies within the system because they often carry a virus. The firewall would have detected the program trying to dump images and alerted the user, who if completely innocent should have then taken action.You would have to actively override this software and consciously access the site to accept and bypass the firewall.
wheresthelight if you work within the care system I find it worrying that you take such a cavalier attitude towards the safety of children and describe concerns as 'histrionics'. All of the posts on this thread have encouraged the OP to work with the police and put the safety of the children first. This is the responsible thing to do and is in no way histrionic.
I will not go into details as its not appropriate but I have come into contact with sex offenders within the prison environment as part of my work and my experience is they are extremely well practiced at deception. Even to the point of self delusion 'it was the child's fault, if they hadn't been there or led me on, I'm the innocent victim'. You cannot allow any doubt, you must find out the real facts of the case and not take their word for it. This process of grading is for the police and criminal court to use to ensure they target the really dangerous offenders given limited resources. All grades are red flags and should be taken seriously not used to minimise culpability. He has accessed up to grade four which would suggest he is working his way up the scale.
Your mother has also been taken in by this man. You are thinking with a clear head, please, please keep going. You owe it you nephew to be on guard about this man. He is not family, you do not owe him the benefit of the doubt. Better safe than sorry.
You owe this man nothing, you owe your nephew everything given that you are now aware there is a problem. You are doing the right thing, do not be swayed.