I always brush my hair as soon as I wake up in the early morning. On the other hand, my sister uses bathroom when she wakes up. I usually assist my grandmother at our grocery shop and my sister sometimes helps her.
Sentence 2: the first predicate is "uses", which is third person sigular. Who uses= my sister, is a noun phrase used as subject and it is also third person singular. The subject and verb in this sentence agree.
The second predicate is "wakes up", which is third person sigular. Who wakes up= she, is a pronoun used as subject and it is also third person singular. The subject and verb in this sentence agree.
Sentence 3: The second predicate is "helps", which is third person sigular. Who helps= my sister, is a noun phrase used as subject and it is also third person singular. The subject and verb in this sentence agree.