Without the comma, a reader may think at first that Meredith waited for Samir and her sister.

A compound sentence consists of two or more parts that could each stand alone as a sentence. When the parts are joined by a coordinating conjunction —and, but, so, yet, or, nor, or for— use a comma before the conjunction to indicate a pause between the two thoughts. In very short sentences, the comma is optional if the sentence can be easily understood without it. Including the comma, however, will never be wrong. (See 46b.)