It must have happened off screen, but this episode spelled out that Otto knew that Alicent sided with Rhaenyra over him regarding the accusations. I can imagine how embarrassed she felt whenever it was revealed to her that Rhaenyra was lying (by omission), and her father got dismissed for telling the truth.
This adds a ton of fuel to that green fire for why she is like fukk Rhaenyra. She sided with the chick and was made to look foolish, naive, and like a traitor to her own family. It reminds me of when Sansa in season 1 whenever she inadvertently sided with the Lannisters. Alicent feels like she partially helped Otto get fired because she put in Viserys's ear that Rhaenyra was telling the truth.
For years after the birth of her son, Alicent has been out there wearing Targaryen colors, trying to support her friend and husband's house, maybe out of guilt, but still dutifully enough choosing them over her Hightower family. Choosing to support Rhaenyra even though all of the andal laws and tradition say that her son is being robbed of his rights. And she feels like a fool for it.
Now she tossing those Targaryen colors aside and embracing her family. I felt all of that emotion in her entrance in that Hightower green.
And she called her step-daughter for the first time
That "sister" shyt is dead