I increase rent every year and I don't have any issues years into it.

If you do have the reserves or don't need them, I would look at moving it to a business CD or possibly re-investing into another property if it gives you enough for the next down payment.

I would recommend you get way into the weeds on your due dilligence- septics, roofs, subfloors, hot water tanks, drainfields, etc. 

For Cousin B, we can look into mortgage options that facilitate the purchase of the 50% interest in the property.

I know of a person who was convicted of manslaughter (he got into a bar fight when he was young and the person fell and hit their head on a counter and died). 

When diving into this world, i don't want to get caught with my pants down! 

I have the cash to put down 20% for the build but I want to keep it to put it into another rental property after this is complete. 

I looked at the best-case scenario for a rental that I am selling (it's under contract) if a cash buyer bought it and placed it into service at a Rent-O-Meter average price, and the annual yield is 6.33%.

My intent is to convert my primary into a long term rental.

You might check into getting a Home Warranty.