How much tax do you owe and how much are you looking to borrow?

Borrow against it as needed for retirement and keep it as a high end rental if they want to vacate it.  

Long story short, any owner-occupied property loan will require the borrower to show income that the lender can determine to be stable and predictable in their good-faith effort to confirm the ability to repay. 

After it’s built borrow on full value and gift the parents back the land value?

I think about this often, its really just running the numbers - if you borrow 100k at an 8% interest rate, will the investment get you more than an 8% return?

That will give incentive to refinance ASAP whereas these 0% loans the borrower has zero interst in refinancing out early.

That means the government has been borrowing my $600k (instead of $1.2M because I don’t live in California.) interest free for more than 10 years.

"Before borrowing money from a friend, decide which you need most".

As you are partnering with your sister, have a legal document drawn up as to what happens if one of you wants to sell and the other doesn't, how expenses will be split along with money you are putting down, borrowing, profits, etc. 

Some lenders offer discounts to return borrowers so maybe look into both flip and DSCR rates when finding a lender.