Inheritance/Estate Tax - Belgium

A sister-in-law of my mother-in-law passed away.

My mother-in-law is the beneficiary of the estate.

My mother-in-law is from Belgium, but is now an American citizen. Her sister-in-law is a citizen of Belgium.

Most of the legal stuff is being handled by another cousin (who lives in Belgium). Aunt Agnes (like “Annyes”- she hated when folk called her “Aggness”, heh), the sister-in-law, had no children.

What kind of taxes should my mother-in-law expect to pay? Do any of you know of any online resources? I’ve googled for a few, but the sites I’ve come across were not very informative.

Any info is appreciated!