This refers to a long-standing legislative feud between the two houses of Parliament. The bill allowing such a marriage was finally passed in 1907, after some half a century of debate. Without such a ban, it was argued, an unmarried woman might be tempted to poison her married sister and then snare her bereaved brother-in-law. A more likely explanation is given by Stedman (273): "Presumably the ban was related to incest -- Hamlet is outraged at his mother's incest, she having married her deceased husband's brother. Interestingly enough, marriage with deceased wife's sister was permissible in the colonies. In Shaw's Major Barbara Adolphus Cusins is legitimate in Australia, but a bastard in England because his father did [that]."