Despite trying to win over Rhaenys, Alicent has no power of persuasion over the Queen Who Never Was. So, she instead imprisons Rhaenys. However, Rhaenys has many allies, and with their help, she escapes into the dragon lair. There, she takes her dragon, Meleys, and together they erupt through the floor of the great hall during King Aegon II’s coronation. So why didn’t Rhaenys kill Alicent and Aegon then and there?
Source: https://www.distractify.com/p/why-didnt-rhaenys-kill-alicent-and-aegon