[5] In 1885 she moved to Amsterdam[6] where she attended the Rijksnormaalschool voor Tekenonderwijs (National Normal School for Drawing Teachers).

There she became acquainted with the work of artists involved with Les XX and La Libre Esthétique.

[13] In 1901 she exhibited her work at the annual Kunstenaarsvereniging Sint Lucas [nl] show at the Stedelijk Museum Amsterdam.

Koster also began a friendship with Lidy van Bosse, who was able to arrange for portrait commissions which provided income.

Two of her works were included in the 1913 show De Vrouw 1813–1913 (Women 1813-1913) in Amsterdam which celebrated the hundredth anniversary of the Netherlands liberation from French occupation.

[18] In the late 1920s she spent her summers in Brittany where she met the New Zealand painters Iva McEldowney and Vera Vial.

In the winter of 1939-1940 four works of Koster's were included in the show Onze Kunst van Heden (Our Art of Today) at the Rijksmuseum.

[2][21][3] Koster was a member of and exhibited with Arti et Amicitiae, the Kunstenaarsvereniging Sint Lucas, the Vereeniging van Beeldende Kunstenaars, De Rotterdammers (Association of Visual Artists), and the Pulchri Studio.

The same year her work was included in the traveling exhibition Het Boereninterieur in de Nederlandse schilderkunst (The Peasant Interior in Dutch Painting).