Welcome @Andrae S WigginsThere are REI opportunities in different markets - markets in the Midwest and Southeast of the country are great - where not only are the purchase prices reasonable for most RE investors, but the homes are turnkey (new builds or completely rehabbed homes, tenant ready, systems 10 years of life remaining on them, property management teams in place) with appreciating home value AND appreciating rent. 

I also figured that when he parents do pass down the road, we could refinance through a bank and pay off the sister her half of the home, while still keeping the house and the rental income.

@Iris OlivasWe have financed single-family homes that were under $100k and we have also bundled some of these sub $100K properties together for commcerical loans.

This is a great starting point for you and welcome to PA my home state!

Most of the stuff you can get at Home Depot or Lowes and get it the next day, so no delays once you decide on something. 

New builds-broke even in the '08 market on two spec homes, built my own home from the ground up twelve years ago(400k in equity). 

But then as the home prices kept going up, they decided to get out of the market, thinking that there would either be another crash or home prices had gotten too high at that point.

We’re planning to rent out our fully furnished 3-bedroom, 2-bathroom home in Phoenix-Metro area from March 1st to August 31st in the future.

Even though you want the ADU to be an investment and not your primary home.

A bus tour drove you around town to look at the homes "hidden" in plain site, which you'd never guess were RAL's.