Nuki : What was your sister doing when you arrived home last night?

Rendra : She …. in the kitchen.

was cooking

This sentence uses the past continuous tense pattern. This pattern is used to express an activity that was taking place in the past when another event occurred.

My sister was cooking in the kitchen when I arrived last night.

or

When I arrived last night my sister was cooking in the kitchen.

S₁ + was/were + verb ing + object + when + S₂ + V₂

or

When S₂ + V₂ + S₁ + was/were + verb ing + object.