Complex sentence: The house was built for and owned by the Browne family, some of whom were wealthy slave and plantation owners in Jamaica, these being The 2nd Marquess of Sligo and his wife Lady Sligo (Hester Catherine de Burgh).
Split into simpler sentences: The house was built for and owned by the Browne family, some of whom were wealthy slave and plantation owners in Jamaica. A prominent slave-owning head of the family was The 2nd Marquess of Sligo and his wife, Lady Sligo (Hester Catherine de Burgh).