To Love, To Hate by Fay Adams

This was Marya, apricot golden, capricious, shameless. adoring and adored.
This was Liz, darkly grave, wistful, the patient moralist, and Marya's lifelong victim.
Between them there was a band that could never be broken, for they were twins, unhappy together and wretched apart. Against the background of Paris in every mood, they quarreled, reconciled, quarreled again - and built up a great international cosmetic salon. Then came Kelly Faulkner, the only man Liz had ever loved - and the only one Marya hadn't.