Back in 2004-2006 my father went through something very similar with his sister and her husband. He was the PoA for my Aunt (his sister) and came to realize that my Uncle's children, specifically his daughter, transferred a large amount of money out of an account, and it was a figure much larger than the number's you have provided this far. She had a stroke and my uncle had dementia along with a few other mental issues. Pain medication, along with other medication was found missing, along with family heirlooms and very valuable jewelry.
He never had to go Rambo, although he wanted to. I can say that he did a lot of "surveillance" with camera's in his free time. Get as much evidence YOU can. I know it sounds obvious, but literally stockpile as much info you can and walk into the situation of contacting Law Enforcement with your guns loaded and ready to be professional about it. That's what he did, and it ended up with the daughter being put into prison for a few years, and her husband being put in for awhile longer for unrelated charges as well. Not all the money was recovered, but it was enough that they could still live out their days comfortably. Not to mention getting a lot of things back that money simply couldn't buy.
Why people choose to stoop to this low of a level is beyond me, I hope this all turns out for the best for you and your family.