A reformed thief decides to help his reclusive brother, court his dream girl, when he discovers she’s the daughter of a wealthy investment banker.

The conflict would be much higher if he's perhaps not a very good thief, in debt to some gangsters, needs one big score ... and then this falls into his lap? Maybe his brother is already thinking about asking the girl to marry him ... and suddenly your protag needs to seduce the girl to get to her father's cash?

You should specify 1) what type of thief the brother is 2) which brother is after the dream girl 3) where is the hurdle to overcome.

I don't see a "likable" character here. Who am I rooting for?

But does the current log line make me want to read the screenplay?

dpg: I guess you'll have to read the screenplay to find out.

Why doesn't the ex-thief go after the girl himself?

(And the implication seems to be that the ex-thief is not so reformed after all, that he's motivated more by greed than altruism or love.)

If the thief is reformed then why does he want his brother ot go after the wealthy girl? Perhaps you should say "former" instead of thief so that we understand that his motivation is the money. "Relationship phobic brother" sounds odd to me since it doesn't really roll off of the tongue easily, however I'm unsure how to remedy that. Also, is it the thief or his brother's dream girl?
Suggested Rewrite: "When a former thief discovers his brother's dream girl is also the daughter of a wealthy banker he will do anything to make sure his relationship-phobic brother gets the girl."
Take from it what you will and good luck!