The story might feel pretty unrealistic but its a fiction after all. Knock me if you find this interesting.
The main character is a wealthy duke in his late 20s to early 30s, at marriageable age and as good a catch one could get. The females are a pair of mother and daughter, from high family themselves but are facing ruination though they try to cover it up. The story starts when they have managed to catch the eyes of the duke and the marriage is more or less set between the duke and the daughter. They are now living the duke's grand country estate. The money is nearly at end but they are now used to living grandly once again with the duke's fund, it is at that moment the duke's true intention slowly comes out.
The mother is sill very beautiful and has a figure to match and one night as they talked privately (the daughter probably out for beauty sleep) the duke let her know that he's a virile man with lots of expectation from his marriage and maybe the mother should know about them to teach her daughter .. some practical knowledge.
So basically the guy would come after the mother first then he would also enjoy the daughter or both.
Feel free to offer any suggestions or show interest.