Sonomi-san is Tomoyo's mother, a successful CEO of a toy company. She's the cousin of Sakura's mother, Nadesico. The two were very close ever since they were young, and Sonomi has never quite gotten over the fact that her very best friend got married. To Sonomi this seemed unthinkable, because they HAD married even though Nadesico was still in high school, and because Sonomi did not approve of Nadesico's choice of husband. Sonomi felt her cousin deserved someone wealthy who would keep in her luxury not a poor teacher. she was very blind to Nadesico's happiness, unfortunately.
Sonomi is very busy with her work. She is very competetive and she always tries to prove that she is good enough. For what? Well, for everything, I guess. Part of this was probably caused by what she perceives as Nadesico's rejection of her. Now Sonomi feels she must prove she is better than Fujitaka, but she finds herself losing (not that Fujitaka wanted to compete with her in the first place).
Whatever the case, Sonomi loves her daughter. She doesn't always show it as she should -- giving Tomoyo loads of gifts instead of spending time with her and showing up late for Tomoyo's activities. But at least she tries. Though Tomoyo doesn't seem too bothered by this, it's obvious that she's VERY happy when Sonomi is home more, even if it was because of Sakura. (If she was some other girl, I guess she would've been jealous. But she's not:)
The moment she lays her eyes upon Sakura, Sonomi is fascinated with Sakura's resemblance to Nadesico...and upon discovering Sakura is Nadesico's daughter, Sonomi develops pure adoration for her daugther's best friend and encourages Sakura to spend a lot of time at their house. Sonomi also shows an special thoughtfulness by arranging for Sakura to meet her maternal grandfather. While this doesn't totally heal the rift between the families which Sakura is too young to understand, it brings great happiness to all of them. There's hope for her yet!