Doubt regarding authorship
You give a partial version of the story using pejoratives instead of trying to stay based on facts. So it is difficult to answer your question with that information.

What we can say is that authorship is something to be discussed with your advisor. It will be his decision, in the end. A good way to help yourself is to work hard on that idea, and clearly make your point. The best way to help yourself be first author is to write the paper, or at least the more significant portions of it! Start already with introduction and methods, and as soon as results are gathered, write it up and then submit it to your advisor. If you have done a large part of the work and wrote the manuscript, you should have no problem being first author.