が is so-called a subject marker. "Indicate the subject of a sentence". And the word いる usually takes が, as Martin says.

"が" represent a subject. If I translate "居間に 父と母が います" into direct English, it says In the living room, my father and mother exist. There are not same Japanese expressions like "I have " or "There are " in this kind of situation. We just say "………がいる" if we want to show somebody's existence.

这个说法是完全正确的，这里が就是一个最基础的There be 举行 PLにが ある　いる的用法