It seems a pretty easy answer:
No I wouldn't get into business with them, and the chance of them succeeding is very,very slim.
If this individual isn't willing to put in the time & effort to take care of you properly, especially if you are expected to put in the time & effort to build the product, then they are unlikely to put any more attention towards client retention, sales, or marketing - all requirements of startup success. Without that due diligence they will likely realize that properly taking the product to market will require much more money and effort than they have avaialable.
Also, on the question about trust: trust isn't just blind faith, it's also founded on a basis of respect and openness. If you trust this person then you should repect them enough to explain your position, and to let them know your concerns. Likewise, they should respect you enough to understand you can't work for them without any assurances. Even if there are ties as close as blood, you should both be open at the beginning to set and manage expectations.
Regarding the opportunity - rest assured, there are many opportunities out there with a huge market and much more organized entrepreneurs.
So there - three reasons to say no:
- Liklihood of success is low
- There doesn't seem to be sufficient respect and trust
- There are many other opportunities