Rowing with the Wind (1988) on Netflix

In the summer of 1816, Percy Shelley, his mistress Mary, and her stepsister Claire visit Lord Byron at Lake Geneva. Byron challenges each to write a horror story, and Mary begins her novel, Frankenstein. She imagines the monster becoming real, and for the next six years, as tragedy befalls those around her, she believes the personification of her imagination is the cause. Against this backdrop, Claire has Byron's baby then is estranged from him and barred from her daughter. Byron and Percy continue their friendship, the one self-centered and decadent, the other wildly idealistic. The Shelleys take up residence near Pisa.