Here are thirteen suggestions for plots that center around the protagonists' family members.

1. Two Words: Innsmouth. Taint.

2. Great-Grampa was a member of the Theron-Marks Society

3. Grampa was member of Delta Green in WWII

4. Three Words: The. Dunwich. Horror.

5. Anyone could be a Dreamer

6. Four people: Charles Dexter Ward, Richard Upton Pickman, Edward Pickman Derby, and Arthur Jermyn...

7. Five Words: The. Rats. In. The. Walls.

8. Any relative can be a Haunted Artist(TM) who gets sucked into Hastur

9. Make a sibling-in-law a serial killer - very appropiate without being CoCish

10. Anyone in the family could be a member of the Golden Dawn or similar organization. Hell, how about just a Freemason?

11. By the same token, can you imagine a sibling or child who's an Uber-Goth who trips over Things Man Was Not Meant To Know?

12. What if a spouse was formerly member of a cult (ala _Lord of Illusion_?)

13. What if spouse suddenly _joins_ a cult? Takes children, files for divorce, etc? Flavor Cult to taste.