Then treat it like a normal deal.

The situation is my moms long time neighbor ~20 years passed away of old age, I didn’t know him personally but grew up just a few houses down and said hello daily on my walk home from school, normal neighbor stuff.

Do house flippers normally just split the profits ?

Get them out, clean it up, renovate, and find a normal renter. 

Some other articles say that I would owe the full capital gains on the investment side (half the unit so it would be half the capital gains normal tax).

Like where I live, it's normal to do one month of rent for first, last, and security deposit. 

The McLean County Area Landlord Association meets at 112 Boeykens Place, Normal, IL 61761 Contact is Marti Ryan President.

A normal title company might vapor lock over a non standard deal like this, but your RE attorney can create all of the required paperwork to get this deal closed. 

That's why I warn sellers that setting too high a price, even if people would normally be willing to pay that much, would be screening out all but the most cash-rich of buyers and thus limit the potential to make an actual sale.

This is the single biggest reason to own both sides of a double unit.The risk you take on is that you no longer share the expense of the normal "shared expenses" such as a new roof, driveway, etc.