Hi Nina.If I were not experienced in land lording and property manager, I would look personally to the extent you want to learn or experience property management versus hiring someone else that you trust to do the job.Yes, the cost for a property manager is important but not essential- the cost between property managers are not double or triple the cost of others (say like attorneys in some areas of law or cars). 

I have one LRT and one STR which are both under our personal names. 

.- Tenant Relationship Management: Property management companies act as intermediaries between landlords and tenants, handling rent collection, lease enforcement, and tenant communications professionally and diplomatically.- Legal and Regulatory Knowledge: Property management companies stay updated on local landlord-tenant laws and regulations, ensuring that all aspects of property management, including tenant screening, are conducted in compliance with legal requirements.In conclusion, tenant screening is a cornerstone of successful property management, contributing to tenant satisfaction, property upkeep, and financial stability for property owners.

and include my tenant's name and this unknown person? 

Our down payments came mostly from our personal savings and earnings.

If you decide to entertain this, due diligence will involve digging deep into the business financials to understand cashflow, solvency, and liquidity, his personal finances, and searching for any liens or UCCs in place. 

DSCR loans offer alternatives for DTI/income and employment, but generally still have strict personal credit requirements. 

I recommend buying it under your personal name.

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.         

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.