Welcome to the conversation. I myself do not believe AG and SIL are perpetrators or involved. However, there must be a motive for what happened and it's probably financial.
Some will point out that there is apparently a great financial disparity within the 'G' family. AG and SIL are also saddled with day-to-day responsibility of keeping an eye on NG. The other siblings live thousands of miles away. Over time, in theory, resentment can build. A resentful person may think it would be nice to get a slice of NG's estate sooner rather than later.
For myself, I think if AG and SIL were involved this case would have been resolved by now.