Sorry for your loss...
When you sell a property - the capital gain is determined as (selling price) - (basis)
The basis is generally a purchase price adjusted by some improvement expenses.
If the property was inherited - as in your situation - there will be stepped up basis - that is a fair market value of the property on the day the decedent died.
Assuming your father and mother owned the property jointly - the basis of the inherited half will be a fair market value, but the basis on the half your mother owned - would be a purchase price.
As your mother will sell the property - assuming she owned it and used as a primary residence at least two years from the last five before the sale - she will be able to exclude the gain from taxable income - up to $250,000 for single and up to $500,000 if she file a joint tax return in 2009.
Let me know if you need any help.