I would be concerned abut this rent roll
Why would the owner be the only one paying $900? There certainly is no guarentee that she is remaining in her apartment. Her apartment is probably only worth $600. How do the rents in this triplex compare to other rents? Compare it to Craigs list and Rentometer and to Zillow??
I would want to base the cost of the house on 3 apartments renting for $600 and with the vacant apartment filled. I would think of this as $1,800 a month rent in total. However with the sister that pays only $525 rent you would have to look at her lease. If she has a 2 or 3 year lease at $525 that would not be a good thing for you as the buyer.
You need a good inspection here to see what major renovations are coming up and what their costs would be. Will this house need electricl up grading? Plumbing updates? How old is the roof and what would be the cost of a new roof?
How much mortgage would the owner hold and at what percentage? When you have all these numbers together take a good look at your position.