Answer: Marlo, the oldest in the family, often reads to her little brother and sister.
Step-by-step explanation:
I'm sorry your question was not answered sooner. I doubt you will still need this answer but for the sake of others I went ahead to answer.
When a subject and a verb are said to agree, it means that the verb is the same number as the subject. This means that if the subject is in plural, the verb must be in plural as well and if the subject is singular, the verb must be singular as well.
''Marlo'' is a singular subject and the verb that would complement this is ''reads'' because it is a singular number as well.