What it means is that almost all of the taxable income will be capital gains, i.e.: 20% tax rate (+medicare on the gains above $200K). It will also probably drag the Social Security payments into the taxable income (at least partially), but most of the tax would be for the house.
I suggest checking with a licensed tax adviser (EA or a CPA licensed in your state) for a more precise simulation, and also to explore potential exemptions, deductions and potentially capitalize-able expenses. Check into capitalized renovations and other capital expenses on the house, primary residence exclusion, potentially stepped up basis since her husband passed away as @mhoran_psprep mentioned, etc etc. A decent professional adviser will definitely help you identify all the points.