The key when answering this question is to make it about opportunity at the new employer instead of disaster at the last employer.
Doing so will elevate the new opportunity and personally connect you to the new opportunity. Focusing on the disaster at the last employer will leave them wondering if you caused it. Why did the last employer not trust you to run the books by yourself? Why did they need a family member to do it? And so on. Even if these are not legitimate concerns, you don't want them to enter anyone's thought process.
It can be a very good idea to be specific when you're answering this question, but you need to be specific about the right things. Again, you don't want to focus on the disaster at the last employer, you want to focus on the opportunities at the new employer. Pick something positive and specific from your current job, and tie it to something positive and specific at the new opportunity:
Well, when I was at Acme Construction, I implemented X new process, which taught me Y. I see from your website/job description that you also do X, and I'm looking forward to the opportunity to contribute improvements towards your process as well based on what I learned at Acme.
Of course, being able to do this means that you need to do your research about the new employer before showing up for the interview. If you can't logically make a specific jump like this, its better to leave the answer generic, rather than fake something specific, which will just feel fake. But if you can do your research and make this sort of tie to an opportunity at the new job, it will show your initiative, your ability to learn, and it will take a potentially negative question and turn it into an opportunity to show off a bit.