Estate Tax

The executor may file Form 4810 for a prompt assessment. Without filing Form 4810, the IRS has three years from the due date of the return to make assessments.

You are liable for your husband's share of income tax on a jointly filed return in the year of his death. To avoid this tax liability, do not file jointly. If you already filed, and the due date is past, you cannot revoke your joint return unless an executor or administrator is appointed after the filing of the return.
These rules are complex. Speak to your local CPA about the Estate and Individual tax return requirements.

The value of a custodian account is included in your taxable estate, if you die while acting as a custodian.

This is referred to as income in respect to the decedent and is reported on the estate's income tax return, Form 1041, or the beneficiary's tax return.