Complex sentence: Stephen Maturin has returned from Spain with his family, but impoverished, Spanish authorities have seized his gold after his activities in Peru, so, effectively penniless, Stephen and his retinue stay at Jack's manor.
Split into simpler sentences: Stephen Maturin has returned from Spain with his family, but impoverished, Spanish authorities having seized his gold after his activities in Peru. Effectively penniless, Stephen and his retinue stay at Jack's manor.