If my dad told me I was going to inherit from him and then a couple years later people who dont own the inheritance started whispering about how my younger brother should inherit it instead, Id start getting annoyed too. Regardless of what I felt prior to getting told Id inherit
Because she wasn't Crown Princess at this time. Plus she wanted a brother so her father would get TF off her mother for 5 seconds and let her live a normal life. It's why Aegon is such a threat when she IS crown princess. Because even at 14, Rhaenyra knew that.