This is not a sizeable loan so the borrower has thrown out terms of paying me $3k in 3 months on the $15k loan.

How can I start building my credit score when I turn 18 as eventually I want to borrow money to invest in real estate.

Evergreen is what bankers call unsecured Usually lines of credit given to business's or investors that the borrower takes out the maximum amount and keeps it out.  

During the active term of the Master line, borrower will have the ability to originate sub-notes beneath it per the terms outlined, provided there are funds available under the Master line’s commitment balance.Any suggestions or comments would be welcomed.Thanks in advance!

So the question is- as an investor, would you rather (if possible) borrow the purchase and rehab money from a private lender at say 10% but your paying interest on the entire amount from day 1?

Child LLC's can readily borrow funds from their parent company with no issues.. 

Meanwhile you’re borrowing at a very low tax deductible rate.

My mom and brothers live together, and they did not qualify for the income threshold, so I offered to become the primary borrower as I have a full-time job with good benefits.

They are basically smaller commercial loans.These loans will not have as favorable terms or rates as your standard conventional loan, but will allow you borrow in the LLC name as opposed to your personal name. 

Where it hurt was my ability to leverage and borrow funds.