Questions & Answers about Сестра работает в аптеке, а я в офисе.
Why do аптеке and офисе both end in -е here?
They are answering the question "where?" to show location, so they are in the Prepositional case. The dictionary forms are аптека (pharmacy) and офис (office). To make the Prepositional case, we replace the final -а with -е for a feminine word like аптека, and simply add -е to the end of a masculine consonant-ending word like офис.
Why is there no verb after а я? Shouldn't it be а я работаю?
You can absolutely say а я работаю в офисе, but Russian often drops the verb in the second half of a sentence if it is the exact same action. This keeps the sentence short and natural, similar to saying "My sister works in a pharmacy, and I in an office" in English.
Why does the sentence use а instead of и to mean "and"?
The word а is used to show a contrast between two subjects doing different things (where the sister works versus where the speaker works). It is often translated as "whereas" or "but", though in English we frequently just say "and". You would use и if the speaker and the sister both worked in the exact same place.
Does сестра mean "my sister" or just "a sister" here?