Learn Dutch: Mijn zuster is danseres. - My sister is a dancer.

It is built from the masculine word danser (dancer) by adding the suffix -es. This is the standard way to create feminine forms for professions ending in -er, such as zangeres (female singer) and lerares (female teacher).

Why isn't it "Mijn zuster is een danseres"?

In Dutch, you drop the indefinite article een (a/an) when stating a profession, role, or nationality directly after verbs like zijn (to be) or worden (to become). Because of this rule, we say is danseres rather than is een danseres.

Is there a difference between zuster and zus?

Both words mean sister, but zus is much more common in everyday speech for family members. Zuster sounds a bit formal or old-fashioned when talking about a sibling, and it is also the standard word for a female nurse.