I think Eloethan has summed up Celia well.
Celia is set in her ways, likes life to be 'proper', rather genteel and ordered. For years she lived a lie, probably showing to outside world what a lovely life she had, nice husband, lovely house, clever and beautiful daughter. All perfect, yet it was a facade because she knew (and kept hidden) that her husband was a serial adulterer and her life was not as she presented it to the outside world. The only way she could lash out was in petty ways, such as refusing to go to her daughter's graduation because she had probably just found out that Kenneth had been unfaithful yet again. Cutting off her nose to spite her face is not unusual.
Now the whole edifice of her world has come crashing down. She probably does feel uneasy about Caroline's wedding, but would have gone and supported them. However, all her past anguish has come flooding back now she has heard about Alan's unfaithfulness to his wife - and his absolutely perfect son has appeared on the scene and we are hearing about him ad nauseum. A transference of all the hatred of Kenneth onto Alan, who just doesn't get it, poor sap.
She is a snob with dyed-in-the-wool old fashioned values but I do feel sorry for her, she is unable to analyse her feelings and work through what is happening as the life she thought she had changes around her.
And she always looks like a fish out of water when surrounded by Alan's family at the farmhouse!
Yes, John is brilliant, so well acted.
It's all well scripted and well acted.