@demitx475 - that is why I asked to experts to re-clarify the answer as the order of events matters.
Their point is that once the asset is in the Irrevocable trust, it is not eligible for 'step up'.
But of course for your father, he passed before the property was placed in an irrevocable trust, so it was eligible for the step up.
So your dad's half was eligible for the step-up, but when you mom passes, the property will not be eligible for the step-up.
