The taxi driver's cousin, Ed, married a woman, Asenath, ouf of Fismouth. You would need to get rid of Asenath.
This is the low-spoiler guide for "Taxi driver's family problem" puzzle in Gibbous - A Cthulhu Adventure. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.