Henriette and Alida de Groot, both born in Antwerp, were deported from Mechelen to Auschwitz together with their mother Emmy Sigrid Alida Winther with Convoy 11, which departed 26 September and arrived in Auschwitz 28 September 1942. This convoy included 1742 persons, of whom were 523 children. Upon arrival 1398 deportees were immediately killed. However, the date of death for Henriette as well for Alida has been established as 30 September 1942, just as from their mother: they have been killed toegether in Auschwitz.
Sources: Cith archive of Rotterdam, family registration card of Christian Valdemar Winther; the dossier of Foreigners of the City of Antwerp, nr. 189717, image 716-729 and the Memorial of de Deportation of the Belgian Jews, among others page 26.