How do I prepare myself to take over my parents' business?
You'll see there that I believe education is the priority when you're young. You'll get so much experience with time that you'll be sick of it, but now is the time of your life when you have the luxury of getting an education. Don't think you won't need it because you have a built-in job, you'll need it and want it that much more. Furthermore, never assume you know the future.
If you don't have good enough grades to get into your favorite or most desired college, get into what college you can and apply yourself. Or do the two-year thing and get your grades up so you can transfer to a four-year college. You'll never regret it.
By the way, maybe as you get further into your education you'll figure out what you like studying. Concentrate on that, and your grades will improve. And you'll enjoy it instead of dreading it. Give yourself a break, keep an open mind on this, and keep trying before you give up.
Having an automatic job in a family business is going to work for you only if you do that job well, the company prospers, and you're glad you're there. It's not a reason not to apply yourself in school.