Learn Spanish (Spain): Mi hermana quiere un piso céntrico que sea luminoso. - My sister wants a centrally located apartment that is bright.

We use the subjunctive (sea) because the apartment she wants is indefinite. She is looking for any apartment that fits her criteria, not a specific one she has already secured. When a relative clause (starting with que) describes something we don't know exists yet, we trigger the subjunctive.

How would the meaning change if the sentence used es instead of sea?

If you said Mi hermana quiere el piso céntrico que es luminoso, the indicative verb (es) means she has a specific, known apartment in mind. She is pointing out a reality: she wants the central apartment (that we both know exists) which is bright.

Is céntrico just the direct translation of "central"?

It translates to "central", but in everyday Spanish, céntrico is the standard, natural adjective used to describe places that are "centrally located" or "downtown". Instead of saying a flat is "en el centro", you simply call it céntrico.

Why do we use ser (sea) for luminoso instead of estar? Isn't sunlight a temporary condition?

While actual sunlight changes throughout the day, being luminoso (having good natural light) is considered a permanent, built-in characteristic of the apartment's architecture (like having large windows or facing south). Because it describes a fixed trait, it requires ser.