I think his family didn't want to buy the abandoned mines because that would indicate that there was something valuable in them (because who wants an empty mine?). So instead they opted for marriage and then requested it to be added into the dowery or whatever (therefore the FL's family gets money, status and connections and they give their mines and some other stuff to show good faith).
And they are all clearly psycho so I wouldn't be surprised if the sis or the brother came up with the marriage idea themselves. From his reaction at the end of chapter 4, I would bet money that dumb idea was his (the brother's).
So he only married her because her family owns mines? But why didn't they just buy it from the family? Also isn't fl from a poorish family and that's why her family wanted her to have an arrange marriage? If yeah than why does he care that she knows that he used her?