Complex sentence: He is last seen visiting her in prison in the late 1990s, by which time Longford is frail and in his nineties, while Hindley is still in her fifties but in a declining state of health.
Split into simpler sentences: He is last seen visiting her in prison in the late 1990s, by which time Longford is frail and in his nineties. Hindley, still in her fifties, is in a declining state of health.