I know u won't get my point, but Alicent should have paid more attention to Rhaenyra and respected her and her father's wishes and not have installed her fucking son on the throne. Things started going south when she married Viserys and when she wanted to strip Rhaenyra off her birthright
1) Viserys didn't talk about "birthright." In the eyes of the smallfolk, Aegon has the "birthright" to rule simply because he is a male Targaryen heir. Alicent's position was vulnerable because Viserys never actually promised Alicent that Aegon would be king but neither did he specify exactly what his role would be and that was the point… all his children wanted his attention and love, and a place to belong. Viserys didn’t know what he wanted. Before naming Rhaenyra his heir, he wanted a son so desperately that he killed his wife for it. His conscience pained him badly afterward, and he decided to name Rhaenyra his heir. Nevertheless, he wanted a young woman to sleep with; he didn’t want a wife, he didn’t want more kids, and he had already named his heir. But he married Alicent and had those kids anyway. He really was so irresponsible and on top of that, he tells Alicent in episode 3 that he thinks he probably was wrong in making Rhaenyra his heir because of his dream/prophecy, so ofc Alicent was worried. Honestly, he could have made his mind up about Rhaenyra and shut up about it. Or he could have actually thought of his other kids and given them some work to do and trained them for some position if he truly cared for them. From the moment they existed, they were his responsibility. And for sure it makes sense why his kids by Alicent, not having known any special love from their father, would want to finally claim a position for themselves.
So, Viserys set the Dance in motion by his choices. If he had supported his Targtower kids and ensured they wouldn’t be threatened by their mad uncle (Daemon; who hated them just because they were Otto's and Alicent's blood) in the succession, then Alicent would have fewer reasons to establish Aegon on the throne.
2) Alicent spent time with Viserys because she had always considered her own self in reference to the Targaryens. What was she if not Rhaenyra’s childhood companion? When Otto compels her to visit Viserys she doesn’t want to. She brings along the book of histories she used to read to Rhaenyra. What else can she offer to the Targaryens? This is what they like: to talk about their legacies and their histories. So she might as well read to him, right? There’s no harm done. And she went. She does the bare minimum whenever they are together: the bare minimum of eye contact and conversation. Viserys was SO PERVERSE that even with the minimum attention from a teenager he wanted to fuck her. Not even make her his wife because he didn’t want another wife. And you see how surprised Alicent is when he declares so matter-of-factly that he will wed Alicent, no questions asked. Because she was the easiest bride-but-no-wife choice of them all. She already had her father in court who was a megalomaniac, and Viserys thought that by marrying Alicent he was giving Otto what he always wanted: greater opportunities for social climbing. So he’d stay out of his way, right? That’s what he wanted all along, right? So I can have your daughter any way I like, right? (Because had he married Laena, can you imagine being so chill with Corlys as a father-in-law? Ofc he wouldn’t desire anything potentially restrictive for him.) And I’m saying that he didn’t want a wife because of what he’d done before to Aemma. He had already made up his mind that (to console his own conscience and to honor Aemma’s memory) he would make Rhaenyra his heir. SO HE DIDN'T NEED ANY MORE HEIRS. Then why does he get married and have some more?? BECAUSE HE IS AN IDIOT and because he just wanted a young bride. He is the reason the war began, and his reckless choices, not Alicent wanting some compensation for her years of mistreatment and suffering. Because even after he got married and had those kids he still had time to make things right but he didn’t.
So Alicent didn’t marry Viserys, it was the other way around. He lusted after her and wed and bed her. Teenage Alicent couldn’t and wouldn't go against the will of the King.
3) Regarding the friendship between Alicent and Rhaenyra it was actually the opposite: Alicent many times tried to defend her and had her back even when Viserys and Otto believed the rumors of her losing her maidenhood (which as an unmarried heir was an insult to the crown). Alicent didn’t want to believe it and she supported her! However, even earlier than that, Rhaenyra cast Alicent off without understanding or offering consolation for the predicament she found herself in. She never tried to understand Alicent and never really paid enough attention to what her friend was going through even before she married Viserys. Alicent was alone and without a friend and Rhaenyra should have paid more attention to her.
4) Things really started to go south when Rhaenyra betrayed Alicent's trust by lying to her and continuing to flout her freedom, special treatment, and life without consequences to Alicent's face. The reason Rhaenyra had bastards and married Laenor was her own fault in the first place. Alicent tried to encourage Viserys to look for any potential suitors for her, and Rhaenyra had every opportunity to make the right choice but no, she messed it up by seeking out Daemon and then sleeping with Criston, so she brought it to herself. After what she did, there was no suitor available apart from Laenor, and her father forced her to marry him because no one else would. And then she had three Strong kids, making every mistake after the other (although I understand her completely: she had to have those kids because she wanted heirs and because that’s her body her choice, you know? She can sleep with whomever she wants) but at the same time, the game they all play is political and during the time and age we’re discussing, it was a big deal if you had bastard kids, especially since there already were THREE legitimate male Targaryen heirs whose fate was unsure and further endangered by Rhaenyra's actions. But it wasn’t Alicent’s fault that Rhaenyra had those kids, it was her own choice and she had to live up to the consequences of her actions. (She did this once by taking her children away from KL because she, too, recognized that they were being pointed at too much and it was getting too obvious).
5) Alicent knew that Rhaenyra wasn’t going to hurt her kids but the system they were born in would. In case she became heir, for Rhaenyra to secure her succession and have no challengers, Alicent’s kids had to die. Those displeased by Rhaenyra could easily dethrone her if there was an active male heir free to roam around Westeros. Alicent's sons would always be a challenge to Rhaenyra. And maybe Rhaenyra hadn’t thought about it per se and I trust that she never had vile intentions towards the kids (despite the deleted scenes where she gives death stares to Aegon) and despite never calling them by their names or developing a relationship with her siblings. Her own council and her uncle-husband would advise her to kill them. Their position wasn’t safe. Viserys could have secured it by giving them land and titles instead of the throne and by having them bend the knee to Rhaenyra but he didn’t. So their position was very much vulnerable because again, he didn’t care at all what happened to them, but Alicent, claiming the throne for her son, and saving his life the only way she knew how, DID.