At the same time, the two of yous would probably need to bring cash to the table to buy out your other siblings. 

The good news is that you mentioned staying longer term. if this is 10+ years or more buying should still be on the table.

See if you can source any info about their housing stipend so you don't overprice, but aren't leaving money on the table either. 

First find a loan officer that will allow you to use one in your current financial position and ask them what your co-borrower needs to bring to the table so you can qualify, then you’ll need to find someone who trusts you and that you trust to co-borrow on the loan with you.

But I think he’s probably been hearing about how hot RE is now, and knows he’s leaving money on the table.

Basically I have a builder who is looking at land to build on and is giving us the opportunity to bring an investor to the table.

If the Real estate agent brought the deal to the table.Then they are a greater partner.Who ever brought the deal to the table should be in control.A lot of a partnership arrangement is who is funding the deal is the most valuable unless the work is funding by another partner and/or that partner puts in more than the funding.From your post, if they don't agree to pay your worth, get different partnersand put in writing everyone's split to the dollar!

I agree with the supposition that 100% occupancy reflects leaving some money on the table.

The last thing you want is for this thing to land on a lawyer's table.

We started out too low and were leaving thousands of dollars on the table.