Making a character sympathetic

The other night I picked up an old mystery to reread. Brat Farrar, by Josephine Tey. She's not Dorothy Sayers, but I do like some of her books, and this one is my favorite. It's a cool setup that would make a good movie. Or remake. Or something. The premise is this: Brat Farrar, backgroundless orphan, returns from America to England and an actor spots him and is astonished at how much he resembles one Patrick Ashby of the horse farm Latchetts. One Patrick Ashby who supposedly threw himself off a cliff years ago because he couldn't deal with his parent's death. One Patrick Ashby who would be coming into his inheritance on his 21st birthday--in just a few weeks. Instead, his twin brother Simon will be filling that place. The actor proposes Brat return as Patrick, say he'd just run away that night (they never found the body, after all), and share the inheritance with him in return for some coaching. It sounds despicable--and yet, as a reader you find yourself cheering for Brat and not Simon.
Why?

1. Brat's got a conscience. He knows it's wrong, and it bothers him.
2. Yet he's got a deep, elemental love for horses, and the temptation to even just see Latchetts is strong.
3. When "his" aunt meets to identify him, he's exactly everything she ever wanted Patrick to be. And Brat, who's never had anyone like a true parent, can't help but love her. It's not money he wants--it's a desperate longing for a place and a family that's truly his.
4. We don't like Simon once we meet him! Brat loves Latchetts like Simon never will. He belongs there.
5. Brat's smart, but he keeps his thoughts to himself. But his longing for something he'd never considered comes through. Sometimes it's not what you say, but what you don't say, that reveals character.

And it goes on (in a spoilerish way, so I'll stop with that.) So interesting, though, how a good author can take even someone in a not very likable situation and make them sympathetic.