Villain motive and hero reaction
Question: In my fiction novel, a young teen leaves her adoptive brother when two knights convince the orphan to join their forces. Once he's abandoned, an evil sorceress tucks him under her wing.
I've had problems forming a motive of his turn to evil, and how he formed his hatred for his sister. And in turn, I've had problems of explaining the sister's inability to defeat her brother and notice her affection to another knight.Answer:
The big question that occurs to me is why the girl decided to leave her brother and head off with the two knights. It makes me wonder if there was a falling out between the two siblings, a fundamental disagreement that made her feel she had to leave and left the brother feeling angry and betrayed (hence susceptible to the sorceress).
This is probably something you need to brainstorm on yourself. Also, I don't know what age group you are writing for. But I can't help wondering at what age the girl was adopted and if the boy had developed a romantic interest in his adopted sister. That might explain his anger at being rejected and her apparent disinterest in the knight (i.e. she's avoiding male attention after the ugly situation with her brother).
However, if this is a book for younger readers, you might not want to delve into such matters. There are plenty of other reasons why the two might fall out while still retaining a measure of family loyalty to each other.