I get up very early at 4:30am. My sister doesn't get up early. She often gets up at 9:00am.
Sentence one: the predicate is "get up", which is one person singular. Who "get up"? is I which is also one person singular. The subject and verb in this sentence agree.
Sentence two: the predicate "doesn't get up" is third person singular. Who doesn't get up? is my sister which is also third person singular so the subject and verb in this sentence agree.
Sentence three: the predicate is "gets up" which mean third person singular. "She" is a pronoun used as a subject and it is third person singular so the pronouns and it antecedent agree.