It doesn't have to be a problem with Skywalker blood. People are imperfect, even Jedi - especially hastily-trained Jedi who have been trained at a much advanced age than milennia of practice has shown to be best. Luke already had those things working against him.

Luke has a soft spot in his personality toward his friends. It nearly led him to ruin in ESB - why wouldn't it work on him again? It's his fatal flaw - all heroes have them. Vader knows that Luke will act rashly when his friends are threatened. But the fact that Leia could pull him back from the brink of the dark side would show that his fatal flaw is also what makes him strong. He was nothing before he met Han, Leia, Chewie, and the Droids - now he saves the galaxy with them in his corner.

I appreciate your feedback, skyjedi.