Currently, I own my primary home and a rental in San Antonio, TX. 

If you can get a better primary residential HELOC rate than you can a hard money bridge loan I would say yes.

I live in my own primary residence and that is fully paid off. 

Will you be keeping or selling your current primary?

We're also starting to scour other forums for possible investments outside the traditional MLS sites, and other areas like upstate NY. 

These properties appeal to students and low-income renters, especially if traditional housing is limited.

Plus the seller will want to increase the purchase price, so they still get their $100k ($100k / (1 - 6%) = $106,383).You state this will be your primary and a rental - so is this a duplex?

Purchase Terms: Clearly outline the terms for your brother to purchase the property from you, including the timeline and price.To minimize the financial burden on your HELOC, refinance the property into a traditional mortgage as soon as possible.                                                                                           

However, consider STR seasonality, local regulations, and management demands, and ensure your equity loan and new primary residence costs fit comfortably within your budget.

An ADU is a secondary dwelling unit located on the same lot as a primary residence.