Do you think Rhaenyra would have killed her siblings or it was mere paranoia on Alicent's side? The book doesn't provide a solid answer for this, and in the show it's clear that Rhaenyra would never harm her siblings.
Hi anon, I kind of went into it in this post, and although that ask was about Jace vs. Aegon III, I think the principle remains the same. In short, no, I don't think it was paranoia, but to understand why, we have to understand why Rhaenyra's brothers pose a particular threat to the stability of Rhaenyra (of Jace's) rule. Keep in mind, this isn't a moral failing specific to Rhaenyra, but simply a byproduct of the conditions of her inheritance.
I don't think Rhaenyra would have wanted to kill her siblings (or their kids), or even have planned to kill her siblings, but I also think that ultimately what she wanted wouldn't matter very much. All it would take would be someone wishing to rise in her esteem claiming that Aegon was fermenting rebellion, perhaps producing a forged letter as evidence, or an eyewitness who would swear that he had been secretly meeting with former greens. Could she risk it? Her brothers are weapons that can always be used against her. And at some point, it would be out of her control. Rhaenyra won't live forever, nor will Daemon, and when Jace attempts to take the throne, with no less than 7 legitimate male claimants alive who would have a claim ahead of him, there are bound to be challengers. The Blackfyre rebellion began with much flimsier pretexts.
We have real life examples of this. Henry VII intended to keep the remaining Plantagenets alive when he took the throne, as long as they stayed loyal. After all, they were his wife's family members, and killing them off would not be a good look. But the remaining Plantagenets would always be a threat to the Tudors. Ten year old Edward Plantagenet, the son of George of Clarence, was imprisoned in the Tower of London for 14 years before he was executed in 1499 for a supposed connection to Perkin Warbeck's scheme. Henry VII finally took action at least in part because he was negotiating a betrothal between his heir and the daughter of King Ferdinand and Queen Isabella. The Spanish monarchs did not want their daughter marrying a man whose succession could be challenged, and so Edward, the strongest claimant at that point, had to go. Henry VII's son, Henry VIII, increasingly worried about the stability of his own succession, became vulnerable to the whisperings of opportunists looking to rise in the king's esteem and eliminate their own political enemies. At this point, the remaining Plantagenet claimants became a source of paranoia, justified or not. The arrest and execution of Margaret Pole, the niece of Edward IV and Richard III, was based upon a tunic found in her home that supposedly represented her support for her son's claim to the throne and the restoration of the Catholic church in England. The tunic was almost certainly planted by Henry VIII's chief minister, the protestant Thomas Cromwell, the same man who orchestrated Henry's divorce from Catherine of Aragon (yes, the same princess whose hand Edward Plantagenet had died to secure). And Henry VIII liked Margaret, she'd been the governess to his daughter, and though they had their ups and downs, he certainly didn't hate her. Still, when her son was put forward as a rival claimant and she was accused of supporting him, she had to go too. And of course, going backwards a bit, there are famously the princes in the tower, Edward and Richard, sons of King Edward IV, who despite having been officially declared bastards (a law, you see, was not enough), were still enough of a threat to the throne that they were (most likely) murdered, whether by Richard III or one of his associates. Mere rumors that those boys still lived sparked rebellions during the reign of Henry VII.
And you can say well, there's a difference, surely, in that Rhaenyra is the rightful queen, and these other people were not? But "rightful" is not some inherent state of being, it's dependent upon who is in power. Every person who sits the throne believes themself to be the rightful king or queen. But Rhaenyra in particular gained her position because her father exercised his power and declared her heir in defiance of the expected order of inheritance, contradicting the very decision that made him king in the first place. After Viserys dies though, for all intents and purposes his wishes cease to matter. He is no longer king, and lacks any mechanism by which to enforce his wishes from beyond the grave. At that point, people will choose to support one claimant or another, based upon their own concerns (dragon math, precedent, oaths, promises made by one or the other, existing family bond) and to consider Rhaenyra or Aegon (or any other claimant down the road) the rightful king/queen. Rhaenyra's security upon the throne, like the position of Henry VII or Richard III, is inherently weaker because she comes to the throne through unconventional means. All it takes is a plague year, a famine, or a foreign invasion for any random group of lords to decide that the true king Aegon/Aemond/Jaehaerys/Maelor should be on the throne and that they should start a rebellion in his name. If Rhaenyra feels insecure in her rule, or in Jace's ability to peacefully inherit after her, it only makes sense to eliminate any potential rivals, and her brothers and their children will always be a threat, no matter her original intentions. Even if Rhaenyra keeps her word and does not harm her family, her brothers and their line pose a threat to Jace and his line as long as both lines exist.
So Alicent is not being paranoid at all, she's being realistic. If Viserys were to disinherit Rhaenyra, or were Rhaenyra to accept the peace terms and give up her claim, she would become simply another sister, but Aegon can never be just another brother to Queen Rhaenyra because in the eyes of some, he will always be a potential rallying point for dissenters, and if not him then his brothers, or his children, whether they want to be or not. That's the point Alicent is making. It's not a reflection on Rhaenyra's character, it's just that if it came down to a choice between securing her reign/Jace's succession, and the lives of her potential political rivals, it's not difficult to guess what Rhaenyra would choose.