I take it "his wife" is not your mother?
Im no expert but at a quick search around the net it appears there have been some studies done on human cremains. Some info I found said that larger portions are necessary to get any accurate information and to my understanding, modern day cremation results in very fine cremains.
As for hoping to find a poison that may have been in the body at the time of death, I see that as highly unlikely. I doubt very many, if any, poisons would leave such a trace in actual hard body matter (as oppsed to blood or soft tissue).
You may have better luck tricking someone into a confession
