It fell mostly on me, I talked my parents into a getting a trust set up so I knew what would be the outcome of the property's and rental business, when they passed, and so did my siblings.

You need a well educated real estate attorney who has many years of experience with real estate partnerships.They will probably advise you to form a LLC (limited liability company)and designate each of you as a partner.I will tell you right now that I don't like 50/50 partnerships because getting into arguments among partners(especially siblings)is too easy.One of you should be designated the leader legally and have the final word on all partnerships decisions.I would advise you to purchase 1% or more from your sister in law and be the dominant partner.It will make things go 1000 times faster and easier not having to run to your sister in law to get her to sign every little document over the next few months and the years to come.Also if she decides to cash out of the partnership she can sell her minority position to you or another investor.The lawyer will help you find a good affordable CPA and new property managers (if necessary).

There wasn't any room to do the court process with the surviving sibling who's share wasn't spread to 17 others. ..

With no apparent takers in sight, and the split-up of the siblings imminent, the idea of adopting them ourselves crept in.

Paul,  have a sibling who works for large REITs and when I asked about the software question one name that came up was Fusion. 

It's also something you can pass on to your kids too when you go (or partner, sibling, or whoever!).

Case in point: I just passed on a loan in which the borrower was buying out siblings on an inheritance so her daughter could live there.

Sellers step-father died in 1981 and the seller doesn't know if he had any siblings or if this was his first marriage. 

When I started asking my family and siblings as well as my wife's parents and siblings in April 2009, everyone said I was crazy.

My parents would routinely come to me for a loan if they needed cash to pay for delivery pizza or money for my siblings to go out with friends.