The first hurdle was convincing them to leave via cash for keys.

The best deal I can find today is one that you buy cash, invest a rehab and when you pull out 75% of your ARV you leave maybe a small amount of money in the deal and you break even (BEFORE DISCOUNTING FOR VACANCY, REPAIRS, etc) Yes that means you leave like 10-25k in the deal and your payment becomes $3000/m and your rent is $3000 a month or slightly less.This is for my local market and for Class B or higher RE. 

You also do not want to be too cheap and leave money on the table.If you do all of that you will be well on your way to being successful.  

I currently have around 100k tied up in sticks right now which I would prefer to leave in and just continue to save with the plan of getting a place in the next few years, I would get a second job to raise the funds.

Use your HELOC to purchase the duplex outright for $118K.Your brother’s $50K down payment can immediately go toward reducing your HELOC balance, leaving a remaining balance of $68K.Create a formal lease-to-own agreement with your brother.

If you are not familiar with lease applications/procedure/ and contracts, use a realtor to make sure you don't leave anything  out that can come back and burn you.

You could try filing a case/judgement against them but I’m guessing they ar leaving the area to screw over the next MTR landlord.

If there is a tenant in the property expect that they will likely leave within 6-12 months after you purchase so you can expect to do a rent turn and budget for that.

So, what does this leave the part time investor?  

The tenant fails to pay their last month's rent and leaves the place needing new flooring and paint.