I don't hate Astrid, this is honestly just a question, but it's said several times that she resents her brother, even hates him a few times. So doesn't she also kind of sacrifice Petey for herself?

I know what you mean :) I’ll try to explain this, but you need to have an open mind, ok? c:

She resented him. He had turned her into a mother at age fourteen, It wasn’t right. This should be her time to shine, to be bold. This was her time to use her intellect, that supposedly great gift. Instead, she was a babysitter.

Ok, while everyone got to play video games, watch tv, have fun or save the town, Astrid had to wipe Petey’s bottom, shower him feed him, take care of him 24/7 instead of being what she really wanted. I mean, I bet she wanted to go out and do something for the kids inside the FAYZ, to be next to Sam in a battle, but she had to stay home and try to communicate with someone whose social skills were non-existent. 

But she had to keep Little Pete calm. That was her top priority, her brother. Her blank-faced, helpless, unloving, brother.

That’s why she resented Petey: he never said “I love you” or a “Thank you for taking care of me, sis.” I’d flip out if I had to take care of a little boy who didn’t do anything but stare at me and ignore me most of the time.

She hated him for being what he was, for being so needy, and she had betrayed him to spare herself. And now she was far more angry at herself than she had ever been at him.

D,: ok, now, see? Astrid does know that hating him is unfair but she was thrown in this dome, filled with kids, coyotes, crazy things, no food and had to play mommy at 14. In Plague that’s why she finally breaks down, all the pressure and knowing that her autistic brother could make the FAYZ wall-the one he created in the first place-disappear (end of Lies) and everything could go back to normal, back to the parents they haven’t seen in over 8 months. 

Plus you got innocent lives taken away, Drake back from the dead and ready to kill, man-eating bugs, more gaiaphage craziness, etc. She had to make a decision: save her brother and let the killings of lots of kids continue, or save hundreds of kids from being eaten from bugs. :/

So, no. I don’t think she sacrificed Petey for herself. If she had hate him so much (which she didn’t) she would have let Drake kill him in Gone.