Complex sentence: Medora had 6 (half?) siblings by her mother Augusta Maria (Byron) and Augusta's husband Colonel George Leigh of the 10th Dragoons, one of whom, Georgiana Augusta Leigh, married Henry Bettesworth Trevanion.
Split into simpler sentences: Medora had 6 (half?) siblings by her mother Augusta Maria (Byron) and Augusta's husband Colonel George Leigh of the 10th Dragoons. One of her sisters, Georgiana Augusta Leigh, married Henry Bettesworth Trevanion.