Warning This Section Does Contain Spoilers
Motive: Sarah was detailed in her statement and signed a search waiver for the PI in Episode 1, which helps her credibility. Her cooperation makes it unlikely that she would have committed the murder.
Means: It is unclear if Sarah has the means to have killed Charlie.
Opportunity: Sarah could not have been at the murder scene at the time of Charlie's death because she was at home without transportation. In her statement, she says that she was watching their daughter all night and that she and Charlie only have one car. These facts are confirmed by:
Crime Scene Description (Episode 1) - Corroborates that the car Charlie died in was registered in his name.
Antonio's Statement (Episode 1) - Corroborates that Charlie left the reunion with someone else in his car, meaning it had to be someone who attended the reunion.
Based on this information, Sarah can be eliminated as a suspect.