I would think transient room mates on renewable short leases would be much safer than bearing the brunt of San Francisco's LTR tenant laws.

This is one way to manage costs risk.The ways to own the property besides FHA & conventional loans is with private loans, investor DSCR loans (non owner occupied); seller financing, lease options, partnering with others to take the property down..... 

It's difficult to see how a tenant will be after screened and signing a lease.

Appraisals, Lenders and Rates : Beyond the transactions that were literally in process (sales, listings, leases, seller carries) are properties that remain amongst vacant lots.

How frequently is the GP on site?  

@Ali Mol In a lease-to-own agreement, the $1,666 monthly credit should be treated as a liability (deferred credit) rather than rental income, as it is allocated toward the purchase price.

The current 99 year lease started in 1987, not 1945.

Alternatively, your sibling might use a 1031 exchange to defer taxes by reinvesting in like-kind properties, though this doesn't directly fund a primary residence.Other ideas include leveraging a HELOC on the rental properties for the down payment, taking out a 401(k) loan, structuring an owner-financed agreement, or a lease-to-own arrangement.

You need lease and deposit check at closing so the property has to be move in ready not a rehab.

Consider hiring a paralegal well versed in leasing, renewing, and evicting.