Complex sentence: Eric, Cecile and the painter Dorothy Johnstone (1892-1980) had formed a close painting and physical liaison, but Eric's excessive drinking led in 1923 to the collapse of the trio and Cecile's moving in with Dorothy.
Split into simpler sentences: Eric, Cecile and the painter Dorothy Johnstone (1892-1980) had formed a close painting and physical liaison. However, Eric's excessive drinking led in 1923 to the collapse of the group and Cecile's moving in with Dorothy.