Learn Arabic (Modern Standard): الأخت ممرضة.

In Arabic, you don't need a verb like "to be" (is/am/are) in the present tense. Putting a definite noun like الأخت (the sister) right next to an indefinite noun like ممرضة (a nurse) automatically creates a complete sentence.

Why does الأخت have ال (the) but ممرضة doesn't?

This is how Arabic builds a basic equational sentence! The subject you are talking about usually has ال (the), making it definite. The information you are giving about it is usually indefinite, so ممرضة doesn't have ال. If you put ال on both, it would just mean the phrase "the nurse sister", rather than a full sentence.

How do we know the word for nurse is feminine?

The word ممرضة ends with a taa marbuuta (ة), which is the most common sign of a feminine word in Arabic. Because "sister" is feminine, the job title describing her must also be feminine. For a male nurse, you would drop the ة and say ممرض.

How would I ask "Who is a nurse?" using our current lesson?

You would use the question word من (man), which means "who" and is used for people. To ask "Who is a nurse?", you just put it at the start: من ممرضة؟. The answer to that question is الأخت.

Could I use ما (what) to ask what the sister's job is?