Here's why:
* Pronoun: A pronoun is a word that replaces a noun.
* Vague: A pronoun is vague when it's unclear who or what it refers to.
* "Sister" is a noun, but it's used here as a pronoun. We don't know which sister the sentence is talking about. It could be the girlfriend's sister, or someone else's sister.
To make the sentence clearer, you need to specify which sister is being referred to. For example:
* "My girlfriend hates her sister but is really nice."
* "My girlfriend hates my sister but is really nice."