see this article about inherited real estate. basically, rev rul 86-139 says you get a step-up in basis for real property inherited from a non-resident alien despite the fact it is not subject to US estate taxes.
https://www.hodgsonruss.com/newsroom-publications-11026.html#:~:text=Rev.%20rul.%2084-139%20%281984-...
what you cite is what happens if mom gifts you the property before she dies. then you get her basis and thus any gain when you sell is subject to US income taxes.
my reading of the rev-rul and the article by the law firm would indicate that if you are gifted d any part of the property before she dies, you get absolutely no step-up. see a lawyer who is knowledgeable in foreign inheritance and gifts. you may want to contact your state bar association to see if it can provide referrals.
