So many factors can affect a parcel's value including topography, soil quality, zoning, history, utility access, road access, shape, etc.

Well, at first glance it appears to be human excrement, BUT the bowl on top of the tank indicates that a person has "created" a mess by filling the toilet and mounding it up with soil from the backyard, which explains the bowl used to transport it into the house.  

@Sandy Uhlmann Hi Sandy!

Sandy,Having parents that have flipped houses and being a GC myself, I feel like I can at least give my opinion from both sides.

In this case, like @Sandy Salazar said, you want the sister to be financially responsible.  

Also, ask the neighbors (if there is any) about the soil.

Don’t think this sand is lingering from construction.

I am under contract for a 72 unit property and we got a bad phase 2... the lead levels in the soil are above the threshold.

I Highly recommend asking if he has any soils reports, surveys and any engineering reports for the property.

I discovered the first home was graded well, but the backfill was soil not gravel.