A groomsman was now also called the "best man." If Sirius got married one day, would he ask James or Regulus to be his best man?
Even housewives had a few friends, yet Petunia had none, because Harry was always causing little "trouble." If she invited anyone to her home, it would be hard to explain things to them, and the "rumors" would spread through the entire neighborhood at an alarming speed.
After getting married, Petunia had no friends. Even the ones she had while working had grown distant after she left the workplace.
A bridesmaid was also called the "maid of honor." Perhaps Mr. and Mrs. Evans still considered Lily the pride of their family. How was Petunia supposed to tell people which school her sister had graduated from? Hogwarts was nowhere near as well-known as Cambridge.
If she told people Lily was a witch, the wedding might as well be called off.
Since Lily was neither her best friend nor someone who could bring honor to the family, why should Petunia invite Lily to be her bridesmaid?
Marriage meant forming a new family, and Petunia was the mistress of that home. When Lily failed to get what she wanted, she claimed Petunia was jealous of her and afraid Lily would steal the spotlight. But Sirius saw it in an entirely different light. Lily, whom James loved beyond all reason, was merely a very ordinary girl in Sirius's eyes, perhaps even a little too meddlesome. After James got together with her, he did not even ride his broom anymore. Without a broom, how was he supposed to go on missions and fight Death Eaters?
Lily was lonely too, both at school and after graduation, not merely during the period when the Fidelius Charm protected her and Bathilda was the