Complex sentence: The 4th and 5th Cavalry Divisions each consisted of three brigades; one a British Yeomanry regiment and two British Indian Army cavalry regiments, one of which was usually lancers, except the 15th Brigade, 5th Cavalry Division which consisted of three Imperial Service Troops lancer regiments.
Split into simpler sentences: The 4th and 5th Cavalry Divisions each consisted of three brigades, one a British Yeomanry regiment and two British Indian Army cavalry regiments, one of which was usually lancers. The exception was the 15th Brigade, 5th Cavalry Division, which consisted of three Imperial Service Troops lancer regiments.