One of my least favorite tropes is "brother's best friend falls for sister." The sister is forbidden fruit. Must. Not. Want. It's an annoying trope, because those involved are adults (unless this is YA) and can and should be making decisions on who they want to date. This one wasn't too bad on that front since Alyssa's brother was not present for most of the book.
I thought Marco's PTSD was realistic and it did Alyssa time to realize that people do change when faced with war. It took her time, but she did eventually "get it." The ending was a bit abrupt I thought.