Basically they'll come up with reasons as to why the "guarantee" doesn't apply to your situation. 

After talking to another family member they said they would be willing to go in on the duplex and offered to pay the $40,000 down payment and help with any repairs but they want to be "partners" or "own" a percentage of the duplex.

Theres nothing wrong with that.

I am probably the guy you want to talk to as I am a Realtor here in Colorado Springs who primarily works with investors, many of those who flip properties.

@Walder JavierRFG is solid, my team and our respective investorshas closed roughly a dozen with them.

I've never worked with them but I recently heard about them through the Milwaukee REI grapevine.

Start with one that might need some clean up and go from there.

I inherited a property with two tenants who don’t pay rent.

I'm right there with you. 

Check with your broker and make sure this fits with your local laws.