です/がいます
I wanted to write: My friend's family consists of his father, mother and an elder sister.
私の友達の家族は父と母と姉です。
The sentence below was corrected by a native speaker. However, he could not explain grammatically why he です was inappropriate and changed into がいます。
私の友達の家族は父と母と姉がいます。
Could somebody explain it for me?