First, let me say going into business with a relative tends to only work well with laundromats and restaurants (and even that can turn nasty, my grandfather had a lot of money stolen from him by relatives on more than one occasion).
Secondly, have you done your due diligence? Market research, project scoping, initial costs, hiring needs, competitor analysis, etc.? If you haven't, make absolutely sure you have a strong business case before you dump all of your money into this. Have you obtained (or do you need to obtain) outside capital? If you're confident enough about your business case to pitch a VC your idea and they're confident enough in it to lend you the money, then that would be another step towards securing option B
Also, how old are you? Let's say the business doesn't take off and you need to trudge along for 10 years before things take off, can you wait that long?