Mass Effect Edit
As with all of her sisters, Sarah had self-defense lessons at the urging of her parents. She took up aikido while her sisters studied other disciplines.
Sarah can be overheard on Ashley's mail declaring Kaidan Alenko (or a male Commander Shepard expressing an interest in Ash) "cute" after seeing the man on a news vid about the Normandy. If Shepard inquires about Ashley's family life, Ash talks about how she bonded with Sarah after being tense with each other for a while.
A couple of years ago, Sarah had a boyfriend named Mike who was very impatient to take their relationship to the next level. Sarah violently objected to Mike's proposal and slammed him face-first into a tree on one of their romantic walks in the woods. Ashley rushed home to Amaterasu and walked Sarah to school for a few days when she heard about the incident, much to Sar's embarrassment.
Sarah told everyone at her school about what happened between her and Mike, making the boy understandably angry. On Ashley's last day of emergency leave, Mike confronted the sisters. Ash wanted to beat the kid, but Sar thought it best if she handled it herself. Sar kept her cool as Mike vented and screamed at her face, deftly evading the boy's punch when it went for her. Mike faceplanted into the floor and injured himself as a result, though Sar helped stop the bleeding and told the responding paramedics that he "fell". Mike finally apologized to Sarah before reaching the hospital, and Sarah accepted it by hugging him.
Mass Effect 3 Edit
By 2186, Sarah married a military man named Thomas, a Second Lieutenant stationed on Demeter. The decision surprised her family, who thought Sarah didn't prefer the military life. The couple were out on honeymoon when the Reapers invaded the galaxy, causing Thomas to be recalled back to service while Sarah joined her mother and two other sisters on Earth. They eventually found their way to the Citadel during the war, but Sarah later learned that her husband was killed in action.
If Ashley survived Virmire, she can update Shepard with news about her family, Sarah included, if the Spectre inquires. After the failed Citadel coup, Sarah prepares a memorial service for her deceased husband at the Refugee Docks' Memorial Wall, and she wants Ash to say some words about the Williams family that Thomas would appreciate. Ashley and Shepard can choose to attend and offer condolences.