Absent any other directive to the contrary, mom was a tenant by the entirety to the house and it transfers to her without any probate or tax worries. She just needs to take the death certificate to the court house to clear the deceased's name off it. Similarly any joint accounts, retirement plans or insurance policies that she is a beneficiary of transfer to her.
There's no Virginia estate tax anymore. Federally, unless the total number of lifetime gifts and the remainder of the estate is in the millions, there's no federal tax due.