The only thing that springs to mind is the Lazienki Park, but that belonged to the Poniatowski family.
They may have lived near this park, in that case they may have lived in Al. Ujazdowskie street, the street that opposes the park from the west. Once the street hosted many residences belonging to noble families (now it hosts many embassies, included the ugly American one which was erected on a place made free thanks to the barbaric demolishing of one such historic residence by the Yanks). But they may have lived near another park, including one which doesn't even exist today.
What seems to be mysterious is the name of the hotel: Mirror Lake Hotel. I can't think of any "jezior(k)o lustrzane" in Warsaw. Maybe "Morskie Oko" (Sea Eye)?
P.S. The £azienki Park belonged to king Stanisław Poniatowski, but later became the property of the Russian Tsars; as soon as Poland regained independence, it bacame public property, I believe.