I usually wake up at 7:00 AM, while my sister often wakes up at 8:00 AM.
I often read for half an hour in the evening, while my cousins usually watches TV.
Sentence 1: The main verb is "wake up", which is first person singular. Who gets up? ="I" is a pronoun used as a subject, and it is first person singular. The subject and verb in this sentence agree.
Sentence 2: The first predicate is "read", which is first person singular. Who gets up? = "I" is a pronoun used as a subject, and it is first person singular. The second verb is "watches", but the subject is "my cousins", which is a plural noun, so the subject and the verb don't agree.