The Stranger's Child by Alan Hollinghurst

In the late summer of 1913, George Sawle brings his Cambridge friend Cecil Valance, a charismatic young poet, to visit his family home. Filled with intimacies and confusions, the weekend will link the families for ever, but its deepest impact will be on George's sixteen-year-old sister Daphne.
As the decades pass, Daphne and those around her endure startling changes in fortune and circumstance, reputations rise and fall, secrets are revealed and hidden and the events of that long-ago summer become part of a legendary story, told and interpreted in different ways by successive generations.
Powerful, absorbing and richly comic, 'The Stranger's Child' is a masterly exploration of English culture, taste and attitudes over a century of change.