Tipping point for characters

This is what marriage was invented for. The Romans married as a way of regulating property and building long term networks. The whole concept of aristocracy is based on marriage, in which both parts held comparable stakes. This can be found all the way down to the middle ages in the life of Eleanor of Aquitaine, who held about a third of France when married to its king and took it with her when she ran to the king of England. 

Compare this to the customs of oriental scum who hold women as pleasure livestock and you'll see an essential difference. As much as GRRM the feminist leftard wants to moan about it (because leftards always moan about the cost of everything), marriage is what made women property holders in the western culture. The idea of marrying for love is just a plebeian romantic fantasy.

Note, by the way, that Cersei's main concern in the matter was not marriage but the consequence of having to leave her children and possibly, although she never says so, her position. This latter point must remain a matter of debate because Cersei never took any actions in early seasons which could not be justified by the wish to be there for her sons. Up to S6, she can be seen as protecting Tommen from Margaery's partial influence, which we know she does not invent as Margaery is constantly shown manipulating the boy, then from the High Sparrow's. 

As such, she is a parallel to Queen Brunhilda, daughter of a Wisigoth king given in marriage to a Frankish king (Sigebert of Austrasia) and who would turn to be the actual ruler through her children and grandchildren after his death. Brunhilda is the stuff of legend, most likely the inspiration for Brünnhilde in the Nibelungenlied, and was made to marry her nephew for political reasons but the marriage was contested by the nephew's own father and king of Neustria, Chilperic. 
King Chilperic, Sigebert's brother and Brunhilda's brother in law, besieged and captured his own son, had him shorn and stuck in a monastery. The son later escaped, sought sanctuary in a church and is said to have committed suicide. Chilperic had taken a common woman as wife, another near legendary figure: Fredegund, who might have killed the former queen. After his death, she became Brunhilda's enemy.

If a TV show were made of the history of the Merovingians, most people would think it crazy and overdone…

I don't think Cersei does hateful things, by the way. She might say a few but always in reaction to some serious threat and she would never do half of what she threatens to. Jaime knows that while Tommen doesn't, which led to that little dialogue in S6E02:

Jaime: Have we caught Prince Trystane's killers?
Tommen: I expect it was Mother.
Jaime: Your mother wouldn't do that.
Tommen: Yes, she would.

This exchange summarises public opinion of Cersei. Young, uninformed Tommen is the average viewer, judging her guilty without cause, while Jaime has the deeper insight and knows better. He knows she was the one to scold him for pushing Bran, who spoke against starting a war with Dorne in S5. Tommen will eventually condemn his mother.