Illustration by Michelle Mildenberg A woman gets engaged and promptly receives a note from another woman who claims to be intimately intertwined with her new fiancé. Scandalously, that woman is the fiancé’s own sister-in-law, and she doesn’t see why her beloved’s new betrothed should tear them apart. What if the couples – the two brothers and their wives – were to move into houses on the same street, and embrace an unconventional attitude to marriage vows? Are we in New York, where polyamory is an emerging trend for young professionals who are unconvinced by traditional relationship models? Or are we on a commune in California in 1967, at the height of the free love movement? Neither. We are in London’s high society, the year is 1887, and the women are married to the brothers of the Conservative politician Arthur Balfour. Their houses – which they affectionately term “the Colony” for the purpose of this ménage à quatre – are Victorian mansions in the heart of Kensington.…