Mother-In-Law

From the moment Lucy met her husband's mother, Diana, she was kept at arm's length. Diana was exquisitely polite, and properly friendly, but Lucy knew that she was not what Diana envisioned. That was five years ago. Now, Diana has been found dead, a suicide note near her body. But the autopsy finds traces of poison and suffocation. Who would want to kill her? And why was her will changed at the eleventh hour to disinherit both of her adult children and their spouses?