He spent it all on...well, I've been cautioned repeatedly not to mention ladies of questionable reputation and a certain white powdered substance with stimulant properties here on Bigger Pockets.Or give it to guru-boy and his shills.

Your proof of funds can be your most recent bank statements (acct. numbers censored out) - otherwise how would the seller know your offer actually has any substance to it? 

(IF) your sister-in-law is dealing with substance/drug abuse of some kind then you could encourage her to check into a rehab then you and your wife could help sort some things out while she’s getting help for herself. 

There was a milky substance spilled in the fridge, some dirty pans and dishes in the cabinets (I just put everything in and ran a load), my girls found an old spilled McDonalds cup under the nightstand upstairs and I pulled a water bottle cap from under the sheets when I crawled into bed last night.

Substance rules over form.

The stew is no longer seen as separate ingredients but rather as a pot of stew being its own form or substance collectively.

http://en.wikipedia.org/wiki/Analysis "Analysis is the process of breaking a complex topic or substance into smaller parts to gain a better understanding of it."

You are like a one person housing development agency, renewing homes and providing quality housing (not any easy task).If someone is just doing a surface flip (just cosmetic shine and not so much substance), I am not sure you help any one (save stimulating the economy some for cost of the floors, counters, and paint).

So the buyer wants to conduct some unspecified, unregulated test for a substance that occurs naturally in the environment - by a company for which no license/certification is required and for a substance for which no 'authority' has determined that some specific amount is 'unsafe'!

I used to work in substance treatment facilities that had houses for sober living. $500/person per month is on the low end.