I'm reading Richard Sewall's massive Life of Emily Dickinson. The lives of the Dickinsons repeatedly cross all siblings dating/sleeping with siblings boundaries (in the way that I am appalled by in The Other Boleyn Girl and Hannah and Her Sisters, which both had the same problem):

Emily and her sister Lavinia were both basically simultaneously chatting up Joseph Lyman; their brother, Austin, had to decide between marrying (he was writing both of them passionate letters at the same time) two sisters who were about a year apart in age--Martha and Susan--they were known as "the Gilbert twins" (he chose the younger, which is an even more unforgivable fault); later, when Austin engaged in an affair with Mabel Todd, she first had an affair with his son. Oh my. This is too much.