D H Lawrence 
The White Peacock 

The novel is set in Nethermere and is narrated by Cyril Beardsall, whose sister Laetitia (Lettie) is involved in a love triangle with two young men, George and Leslie Temple. She eventually marries Leslie, even though she feels sexually drawn to George. Spurned by Lettie, George marries the conventional Meg. Both his and Lettie’s marriages end in unhappiness, as George slides into alcoholism at the novel’s close.