In Sisters, it's likely Amara has Asperger's syndrome. She's not very social to people, has lots of knowledge about animals, rather be alone, and is an amazing artist to the point it’s almost as good as Raina’s.
Her not getting a diagnosis also makes sense given that Amara is a girl - Autism Spectrum disorders frequently go undiagnosed amongst girls, especially given the timeframe the books take place in.
Sue taking only Will with her when she hitchhikes to try and get their van fixed, leaving Raina and Amara behind in the van on a summer day in Nevada which is mostly dry and arid as well as vulnerable to anyone showing up to potentially break into the van and rob/assault the girls. It's also implied that there's not much food for them as well...
It only gets worse when you remember that Sue states the nearest town is literally hours away. So in the likely case of something going awry as stated above, things could’ve easily gone from bad to worse with no help for miles.
