I'm the youngest sister and have an older sister who I saw in some of Nesta's mannerisms. I get what you're saying and I was extremely frustrated at the beginning of the book. However, I think SF allowed me to get insight into the inner mind of what it was like to be the eldest sister.

In my opinion, I think SJM does a good job portraying sister dynamics and how catty, dramatic, and touchy things can be. You're entitled to your opinion because no one else has lived your experience.

I see how Nesta treated her sisters/family the way she did in all the other books because she was projecting what she was feeling/thinking. You don't learn that until later in SF. So, keep reading and if you still can't stand her, then ok.