The siblings are the surviving parties.

A few points, the return has to be better than a block home, especially with the older units; this is because when Irma or her siblings come through you have more risk.  

In return once the property sells, she would get the loan amount back + 20% of the profits.My siblings and cousin(her daughter) would like to start an investment company.

So, I have some friends (they're all siblings & motivated sellers) who in the past have talked about wanting to sell their grandparents single family home and their multi-unit property.

He told me  there are 3 siblings total including him, and he has no contact with or has any contact information what soever of the other 2 siblings. 

There probably isn't.All three siblings would have an interest in the house if the parents died without a will. 

Yes the person I talked to is the executor of the estate, not sure if the other siblings are on the same page, that will definitely be a question I’ll ask in the future.

   I have an opportunity to purchase my late parents property from my two siblings who want to sell the property by me buying them out.  

They were basically accidental landlords, who worked hard at their other professions, who were lucky to have good tenants.These building will pass down to the sibling ( my friend) and his brother  as inheritance and will be 50/50 ownership. 

Whomever filled out and filed the document (sibling, best friend, etc.,) will be your first point of contact.Then send off your intro/offer letter @Michael Gessner