I'd say that an action for an accounting seems like the right idea at the right time, from what you've written.

The fact that sister-in-charge won't open up the books is bad enough.  But for her to say that she's owed $16,000, which I'm assuming is the first that any of the rest of you have heard of this at all, after 5 years, simply isn't believable.  Unless you signed some sort of contract that says otherwise, the law won't make you simply take sister-in-law's word for things, especially on these facts.

If you do not believe the accounting, you should require her to substantiate all income and expenses with verifiable reciepts.  It is hard to believe that suddenly there is a $16,000.00 deficiency that she never mentioned before.  Please contact me if you would like my assistance - johntatone.com