He was the only brother of Prime Minister William Cavendish - Bentinck , 3rd Duke of Portland .
Bentinck was the second son of William Bentinck , 2nd Duke of Portland , by Lady Margaret Cavendish Harley , daughter of Edward Harley , 2nd Earl of Oxford .
Bentinck was the only son of Portland   Portland       , 3rd Duke of Portland .
Bentinck was the second son of William Bentinck , 2nd Duke of Portland , by Lady Margaret Cavendish Harley , daughter of Edward Harley , 2nd Earl of Oxford .