Don't have to be related. this is what the IRS says.

The basis of property inherited from a decedent is generally one of the following:

• The fair market value (FMV) of the property on the date of the decedent's death (whether or not the executor of the estate files an estate tax return (Form 706, United States Estate (and Generation-Skipping Transfer) Tax Return)).
• The FMV of the property on the alternate valuation date, but only if the executor of the estate files an estate tax return (Form 706) and elects to use the alternate valuation on that return. See the Instructions for Form 706.