My sister has two children, so she is busy with them in the mornings. I leave for work at 7:00am. She never goes straight to work. She goes to my parents house first. She always helps them because they are very old.
Sentence 1: The subject "sister" was replaced by "she" in the 2nd part of the compound sentence. She refers to sister.
Second and third sentences: Pronoun "she" was still used in the sentences to refer to the word sister, and so to avoid repeating the noun sister in the paragraph.