Owning a duplex at 20 is a huge accomplishment, and it’s inspiring to see you’re already planning your next move.From my perspective, real estate investing is a long-term game focused on building equity, paying down debt, and maintaining stability over chasing short-term cash flow.

Property taxes creep up, exterior repairs , and a brand new HVAC last year wiped out $11k of reserves.

There are tons of other metrics to consider such as rental rate, job demand, growth, path of progress, taxes, schools, crime, and on and on.

Your not correct under many different circumstances.First property tax is 1.1% (fairly safe) with 2% annual increase (prop 13).Next expenses include a lot more than property tax and interest. 

If not, these are liabilities and wouldn't advise adding debt.

Eliminate debt, establish a budget, and save.

@Bradley Buxton makes a very good point to make sure to do your due diligence as you won't be an owner occupant like your in-laws.Last bit of advice: Because this is a friendly deal, you want to take your time to make sure you structure it in a way that maximizes tax and cashflow benefits for both parties. 

As a contractor I had to work for money, be at the mercy of the customer, jobs and timelines never seemed to go as planned, only to pay tax on the money I worked for.

Warning:If you depreciate a property down near zero and then have to sell your property at a loss during a situation of distress...You could end up giving all the proceeds to the bank AND owe the IRS a big chunk of money for recapture.Long-term tax planning with real estate needs to be coupled with risk management and making sure you don't lose any properties.

We are also reviewing a company that can use it to scan bank statements and tax retturns to quickly sort where money is spent and if documents are fraudulent.