My sister and her husband will be moving to Austin for his job next year, and thanks to my constant chatter about BiggerPockets and real estate investing, they're interested in house hacking by buying a small multi-family and renting out the other unit(s) to cover their living expenses.They aren't on BP, so I'd like to help connect them with a few competent, investor-friendly real estate agents in Austin who have some multi-family experience.

There is always something to learn and things are constantly changing. 

The borrower's main assets, I believe, would be his income stream from and partnership in another successful business.How would I go about exploring this option?

Before pulling the trigger to incur all those taxes and penalties, keep it, invest it wisely, and find other sources of capital to fund your non retirement account income stream.

Properties that would provide a steady stream of income, but not transition to a future home for myself. 

Therefore, having a current revenue stream will turn to a cost, because I will need to have the property managed.

Cities are constantly reflecting and changing VR regulations (at least here in Southern California).

We would suggest constant communication and review of budget status as you go along- weekly at minimum.  

You will have constant turnover.

why would you want to buy a terrible property where you are going to have  high probability of constant issues long term?