Book Alicent was completely against Aegon and Helaena’s marriage.
In Fire & Blood, Alicent actually begged Viserys not to marry Aegon and Helaena, calling the union a sin and warning that it would bring ruin upon them. She even told Viserys: “You have brought hell upon them.” It’s a striking difference from the show, where Alicent’s feelings and motivations around her children’s marriage are portrayed very differently.