Nobody has a "right" to use a specific product or service.

I, personally, would consider any other option, including bankruptcy for your sister if she really has that much debt that she can't pay, rather than ask my nephew to give that much equity away and start over smaller with a much higher rate.         

I recommend buying it under your personal name.

Based on what you are saying it causes me to question whether or not the end product has the value that you see in it.

When the first type of person chooses to self manage, they generally have a great experience, and they also get to pocket the extra 20ish% that they would pay if they hired someone else to manage their STR.

With buying a home for personal use, what are the best months of the year to purchase where the cost will be lower.

Don't just spam blasts to agents, take a personal approach.

Other ideas I've seen include taking white label products and putting your STR branding on them to sell.  

We spoke with the unwanted guest personally and he said he would leave by Aug 15.July comes around and the main tenant has decided not to pay the July rent.

Just a piece of advice if you do elect to pull some cash out is to stay at 60% loan to value. 60% loan to value for most products will get you the best combo of rates/costs on the loan.