These were the times

In order to understand the compromises and accomodations made by the two female protagonists in this film, one needs to look through the eyes of the era. Few marriages were made for love. They were made to unite aristocratic families, their lands and their wealth and to preserve it through their heirs. The women who married into these familes had one major duty: to provide a male heir to carry on the title and inherit the land and the wealth. Their lot was not to be envied. They had no power. Their husbands had the power to cast them out and take away their children, and yes, if the husband was as powerful as the Duke, to ruin their lovers.
Women have always found a way to survive these situations, not always happily, but at least to survive. Georgiana was fortunate that her husband's mistress, Bess, remained a friend. What she could do for Georgiana was limited, but within that limitation, she did her best. She and the Duke had a romp that turned into a permanent relationship, but she does not seem to have been in love. Through her liasion with the Duke, she was able to regain her children, which her husband had taken from her. Gratitude, a lack of other choices, and no doubt the perks that went with being his mistress kept Bess in the relationship, but Bess was more of a friend in some ways than Georgiana's own mother. And she didn't gloat over G. She was not proud of her situation and she tried to remain G.'s friend. That friendship was shown near the end when flying in the face of the Duke's anger, she insists on accompanying Georgiana to the country for her confinement and to the assignation to give her newborn baby girl into the hands of the Grey family. Bess knew what it was to lose her children, and she had compassion for Georgiana's plight. She had told Georgiana that no sacrifice was too great for one's children, and Georgiana learned by hard experience the truth of that statement. Some aristocratic husbands were in many ways uninvolved in their wives' lives in this period aside from impregnating them. If the wife was lucky, she would have a good woman friend with whom to share her sorrows and her joys. Women got women through. To our eyes it is an odd relationship, but Georgiana and Bess were women and mothers with many of the same sorrows and the same lack of choices. What they did was make the best of a bad situation. Making the accomodation they did allowed both to raise their children in comparative comfort. If you think about it, the wife having to put up with only one mistress, and that mistress a friend, instead of a succession of women, some of whom may have been openly hostile made life more bearable in a time when married men were free to take mistresses. This kind of accomodation was not unusual back then. The Empress Elizabeth of Austria was said to be on friendly terms with her husband Franz Josef's mistress Katharina Schratt.
At the very least, aristocratic wives were expected to look the other way.
Of course, looked at through the lens of my modern female sensibilities, the situation would have been untenable. I think I would have gone mad in Georgiana's place. I'm so glad we live in modern times.