I know that the general guidance is that emergency funds aren't to be invested, but given our financial security and the fact that we already have a personal e-fund in a safe, non-volatile location, would it still be crazy/unwise to invest this 15k in an S&P500 index fund in a brokerage? 

I have no intention to represent sellers or buyers, or ever list a property.

I googled it and found this:If you rent it to tenants, you would treat it as a rental and report it on a Form Schedule E.

If E has any priority over B, for instance, I would not invest in this structure, given that my bank account puts me at a Class B share (side note, their Class B is a little more GP friendly on the surface, than other groups, but devil is in the detail).  

I am very familiar with how to list a traditional, non-furnished house for rent on Zillow.

For Conventional, you can only use 75% of the gross rent until you have Sched E history for the rental for a net rental income analysis. 

@Angelo Llamas If your cousin rents out the property, it becomes a rental once it's "placed in service," meaning available for rent, and he must report rental income and expenses on Schedule E.

No such requirement for improving rental properties and deducting interest on Schedule E. 

Quote from @Rebecca E.: @Katie MillerI came here because I just had my yearly with my new accountant.

Proceeds to segregated account to be disbursed ONLY on rental property improvements and expenses to preserve the deductibility of interest on your Schedule E.