Complex sentence: Britain objected, on principle, to the suggested concerted action against the Neapolitan Liberals, and therefore sent no plenipotentiary, but was represented by Stewart, ambassador in Vienna and the half-brother of the Foreign Secretary at the time, Viscount Castlereagh.
Split into simpler sentences: Britain objected, on principle, to the suggested concerted action against the Neapolitan Liberals. Therefore she sent no plenipotentiary, but was represented by Stewart, ambassador in Vienna and the half-brother of the Foreign Secretary at the time, Viscount Castlereagh.