Complex sentence: Most of these estates passed to Sibyl's husband, Miles de Gloucester, 1st Earl of Hereford, as her dowry, on their marriage in the spring of 1121, which had been arranged personally by King Henry I of England.
Split into simpler sentences: Most of these estates passed to Sibyl's husband, Miles de Gloucester, 1st Earl of Hereford as her dowry. Their marriage, in the spring of 1121, had been arranged personally by King Henry I of England.