The Crystal Key by Patricia Rice

Take one dead FBI agent, one ghostbuster, one skeptical lawyer, add an octogenarian mob, and stir. . .

Fraud lawyer Jax Ives and ghostbuster Evie Malcolm Carstairs want to take their relationship to the next level, but their increasingly hectic lives intervene.
Evie’s Sensible Solutions Agency first big job is to determine if a recently deceased FBI agent financed her retirement with computer crime. Evie’s eccentric hacker team talk tech, but only Evie can hear the haunting tale the agent’s ghost has to tell. Soon she and her team are investigating potentially murderous octogenarians—despite Jax’s warnings.
But when Jax’s brilliant, OCD, and deeply introverted sister shocks him by helping his anarchic best friend uncover the dangerous mastermind behind a phone-scam targeting senior citizens, Jax has to add his legal expertise to avert disaster.
With ghosts and senior citizens demanding justice, antediluvian conmen on the loose, and Evie, Jax, and cohorts stirring the pot—the apocalypse might be safer than the impending granny revolution!