First, I apologize in advance for my coarseness.
Please tell me if I am understanding what you have told us so far...
You didn't have to put in serious, consistent work to get in shape in the first place. You haven't worked out in a year. So because of all this, you want to coach your GF's brother, when all you had to do was depend on your good genetics and inconsistent training? Plus, you want to create an app that coaches others into a healthy lifestyle?
I think the app sounds like a bad idea for you, unless you can get some seriously knowledgeable people on board. Make sure to tell people to do what you say and not what you do. Hopefully your GF's brother benefits from your nudge in the right direction and outgrows you quickly.