I'm starting to think there's one of these people in every family.

In my family there was a great-aunt whom I never met but she was a legend. Every time there was a death there was great aunt Alice, first on the doorstep and grabbing anything of value (family heirlooms, such as they were). Later on another aunt (my mother's sister) was showing signs of being similar but not nearly as bad.

I agree with hollym. This probably isn't a mental disorder. It may be just plain greed here. Plus excessively bad taste in the way she handled the body.

I'm so sorry you've had to lose your mother-in-law--and have so much added unpleasantness.