This is chimarrão, a traditional drink in the South of Brazil, made from “erva mate,” scientific name Ilex Paraguensis.
While the origin of “erva mate/yerba mate/chimarrão” is disputed, most agree that indigenous people from South America (in the region that is now Paraguay) were the first to infuse it.
I adore the ATC for your mother-in-law. (I loved Porto Alegre. And the special cups and straws for mate.)
I was going through my photos recently and came across this one I took of the sign for the men’s room in a public building in Porto Alegre - someone had a great sense of humour! (Sorry 'bout the poor photo quality…)