Complex sentence: Henrietta tells him that although Sam was convicted of killing her brother, it was she herself that shot him dead during a violent confrontation after she and Sam the stable boy, overcome with desire, had eloped and got married at Gretna Green.
Split into simpler sentences: Henrietta tells him that although Sam was convicted of killing her brother, it was she herself who shot him dead during a violent confrontation. She and Sam who then had been the stable boy, had been overcome with desire, and had eloped and got married at Gretna Green.