Why does she say "real" here? It seems just saying "sun's getting low" already mean sunset.
You are correct that saying "sun's getting low" would mean sunset, however she is using "real" as an intensifier. It is common for 'real' to be used this way, particularly in 'American English spoken' Most could consider the use of "real" in this way to be informal and more suitable to speech than writing. It is used as an intensifier only and is not interchangeable with "really" except in that use. Ex: It got real cold yesterday. Ex: You must be real tired.