Sentence:  Instead she arranges, offhand, a nice little farewell dinner for her husband that was and the lady, Flora, who is to be her successor three months hence. ... Flora is generously constrained to leave the sometime-husband and wife to talk the matter over. "Are we not <WSD>wives-in-law</WSD>?" says Madame. And the upshot is that although Flora interrupts them by phone from her flat below several times until the receiver is left off, Madame easily wins back her husband. Indeed, so complete is their absorption that they have forgotten completely the trifling circumstance that they are no longer man and wife. 
Meaning of the word marked with <WSD> tags:  Another wife of one's husband. Typically used in cases of divorce and subsequent remarriage for one's husband's ex-wife, but sometimes used in polygamy for a co-wife.