There is no inheritance taxes on the federal level.
As a recipient of an inheritance - you do not need to claim it as your income. Please see for reference IRS publication 525 page 33 - http://www.irs.gov/pub/irs-pdf/p525.pdf
Only if the money are in tax deferred account - 401k, IRA, etc - the distribution might be a taxable income.
Otherwise - your only taxable income would be the interest credited on the account after your mother died.
Let me know if you need any help.
You are correct - there is no income tax liability.
Washington does have estate tax, but your mother's estate is not above the
$2,000,000 limit -