When I write my sentences, pronouns I use after a conjunction always refer to their corresponding parts in the original sentence in such a way that Original sentence: Mother (subject) Slapped her daughter (predicate) Clause: Because (conjunction) She (subject) Was drunk (predicate) With the analogy in consideration, 'she' would refer to 'mother' Gramatically however, it could refer to any of the two. The analogy I used is just personal preference.
Think of it in this way, if you reverse the sentence, it would be: She was drunk so the mother slapped her daughter. There are 2 pronouns in the sentence. She and her. Surely, 'her' refers to 'mother' (her daughter = the mother's daughter). If 'she' refers to 'daughter' the pronouns would be inconsistent and confusing.
Could be either, due to the sentence not having a precise meaning. Therefore, everyone will have a different interpretation depending on the way they see the sentence.