Most 401ks give you 30 or 60 days to pay back a 401k loan or they count it as a withdraw and that is a taxable event. 

The most important rule is to have the cash flow of the new property be able to pay all its expenses, mortgage, AND the line of credit back.

This is my third year investing with my mini me 12 year old and this is our 5th deal.My 12 year old found us a great deal- I teamed up with my friend and family member agreed to pay purchase and renovations.

But this was an awesome opportunity for me because this is my first living situation where I will be on my own (and getting used to paying a rent/mortgage) but this will put my overall mortgage on a 400k house at roughly $2000 a month for the first year!

The struggle comes when you plan to sell, pay close attention to how the bank would treat the loan in the event you would sell.Even though its a bit more work, banks are typically open to these loans as it provides them with additional collateral in the event of default.  

Or just pay your sister a finders fee for getting the deal?

This creates two loan payments ($100,000 of equity and $300,000 on the new mortgage).Key NumbersHome Equity Loan Interest Rate: 6%Mortgage Interest Rate: 7%Rental Income: $3,000 per monthExpenses (management, taxes, insurance, maintenance): $800 per monthIncome and ExpensesMonthly Rental Income: $3,000Monthly Expenses: $800Monthly Mortgage Payment: $2,000ExplanationThe investor earns $3,000 in rent each month.They pay $2,000 on the investment property mortgage and $800 on other expenses.This leaves $200 profit each month or $2,400 per year.However, you have to pay $6,000 interest on the equity borrowed.This leaves you with an annual loss of $3,600.While the rental property generates positive monthly income, the interest cost of borrowing the initial $100,000 results in an overall loss.

Or, if you pay 20k over list price on a property that is 50k underpriced, you still got a hell of a deal.

See if you will have to pay for recapture.  

Our tenants pay $2,700 in rent and the mortgage+fees comes out to $2,360.