Locke is a good guy! I think he's the only one that gets (understands) the island. He gave the kid the oinment on his open wound which allowed him to halucinate. It was sort of like a First Nation Peoples vision quest kind of thing like using Payote. It allowed Boone with Lockes help to realise what he needed and that was to let go of those feelings for his step-sister.
As for the sister/brother thing they are adults and only step siblings there is no blood relation so it doesn't bother me
