Honestly, I keep circling back to inheritance being the most straightforward one, but it's almost too common to be interesting anymore. It’s not just about the money itself, though. It’s about the perceived right to a legacy, which can twist people into doing awful things to their own blood. I read this one novel where the eldest son tried to blackmail his sister with old, compromising letters to get her to sign away her share of a family business. The motive wasn't that he needed the cash—it was that he felt the business was his birthright, and her presence as a shareholder was an insult. That sense of entitlement, of something being 'owed,' fuels so many of these plots.
Then there's the blackmail to maintain a façade. This one hits different because it's so much about social standing and shame. Think about all those stories where a parent discovers their child is gay, or in a relationship with someone 'unsuitable,' and threatens to expose it unless the child falls in line. The motive is control, but also the preservation of a family's reputation at all costs. The blackmailer often believes they're acting for the 'greater good' of the family name, which makes them a uniquely frustrating antagonist. You get this horrible clash between personal truth and collective image.
A motive that doesn't get discussed as much is simple, vicious jealousy. It's not about money or status directly, but about sabotaging someone else's happiness because you can't stand to see them have it. A sibling who feels overshadowed might dig up a secret mistake and hold it over the 'golden child' just to watch them squirm and lose their shine. The payoff is emotional, not financial. It's pure spite, and it makes the fallout so much more personal and raw. The resolution often hinges on the blackmailed character finally realizing they don't deserve to live in fear of someone else's bitterness.