It has a separate 2 bedroom cottage that is detached from the regular larger home.

You could contact the brother and let him know you intend to empty his apartment, clean it up, and rent it for a much higher rate.

I'm talking about BRRRR'ing properties to the regular person...who may not have very much.  

One way to check is to look for a sewer "Clean Out" cap. 

When doing the DTI calculation would a lender include money that is received regularly from things like a Real Estate syndication, Mortgage note fund and/or other investments?

@Bruce Woodruff They have a 4 year of clean track record, so I’m hoping.

What was the one thing that really made you successful that you could duplicate regularly?

I would not pay attention to that data.1) The owner is probably getting called regularly, and being asked if they want to sell. 

Back then many PM companies worked out a "family" type deal; the janitor got the basement apartment rent free but always had to be on call and the wife was expected to do her share by cleaning the hallways on a weekly basis (this was fine with mom; she LOVED to clean). 

It is up to you, but I would at least charge her for the time it was vacant and any costs you had as a result of listings and cleaning.