The below terms are near infinitely flexible from what I have below, but the idea is that you and Sis only share in the future profits - no part of Dad's original appreciation. 

Each to their own like you say but appreciation is where the money is made in (finite) and (physical) assets.

However if I do a BRRRR I can achieve infinite return by extracting all of my investment.

Chase cash flow so that you can get infinite returns and you will be poor.  

In terms of ConC over my 25 years, my return is infinite.

However if I do a BRRRR I can achieve infinite return by extracting all of my investment.

@Adrian SmudeI would agree the days of buying and getting 100% of your money out so you have infinite returns is not coming back anytime soon but the method still is common you just have more money in the deal if you want it to cash flow.I do agree BRRR is easy to do with super low rates and increased rents for someone who could value add a property.

The work is more than a rehab, but if i brrrr I can achieve infinite return (this is how I made most of my money in RE), but an ADU generates poor return (none until the initial negative equity position is recovered). 

Personally I would leave the 15k in the property and have more of a cashflow cushion while also having an infinite return on your money. 

I have both and LTRs are infinitely less work and less headache.