How long to you need to retain tax records after a death of both parents? Is it seven years from the most recent death?

Seven years is sufficient for any circumstance. For most situatiuons, the IRS suggests three years.

Keep records for 3 years from the date you filed your original return or 2 years from the date you paid the tax, whichever is later, if you file a claim for credit or refund after you file your return. Keep records for 7 years if you file a claim for a loss from worthless securities or bad debt deduction.