What do you think of this?

You have a good idea but you need to think more about setting, will this fan fic be set in the old republic, the new republic, the empire or the post - empire star wars universe? You also need to decide whether the jedi will be apart of the Jedi order or a lone jedi master or a possible jedi exile. Personally I would suggest that you set the book at the fall of the Republic and the dawn of the Empire. You could have the extinction of the Jedis as a opening to the book and you could reveal that this lone jedi master is an exile and being systematically being hunted down by Darth Vader and the 101st. This jedi master could be living as a hermit in a secluded location like Obi-Wan Kenobi and he could be wathcing over this family on a remote moon surrounding Namoidia. The two siblings could be twins with deep set anger and real issues and they could be orphans. But you have to realise that this story line has already been explored in the Eragon books. Another suggestion, the two siblings could be brothers and one chooses the dark suide and the other chooses the light side. Both are force sensitive but the younger brother joins the empire and hunts down and kills the jedi master setting up a titanic showdown between the force opposed siblings of' 'prophecy'.