I am agreeing with Mike and Bill here. Competency was the first concern that popped into my head. I say this with experience as my mother is technically incompetent, however, not legally incompetent as of today. Like any caring family member I would question any large decisions like this coming from her at this point.
Not to discourage you from deal-making with with of age adults. Just watch the length of your deal, disclosure, make double sure there is complete understanding and make sure that you have a lawyer draw up your paperwork for this, even if you have standard forms that you already use. In all seriousness, if this person passes on and you still have a balance you want to make sure your deal is secure.
I think bringing the family in would be like asking a bank for a short sale at below listed value and then calling your end buyer right in the banks face and setting up a double close for a $100,000 profit.
If you bring the family in and it is ever in question that she is incompetent then you could have a situation where someone could say well if you thought she was competent why did you invite her family to see if it was okay? Not to mention that it could possibly be a slap in the face to the current owner.
Or I could be wrong. These are our personal opinions, I am not a lawyer. In a nutshell, consult your lawyer to get the best answers.