Despite being the older sister, Petunia did not like to share. Thus, when her younger sister Lily, the emerald gem of her parent's eye, also got the fated letter she was pissed. She had to share her parents with her sister and now she had to share the magical world with her sister. What was the universe coming to?
The Sorting Hat did not help matters as Petunia Evans ended up a Muggle-born Slytherin in the worst time period to be one the previous year. However, this year Miss Lily "can do no wrong" Evans ended up in golden Gryffindor and she was stuck with dirty, homely, poor Severus in tarnished silver Slytherin.
Petunia still had a grudge against Severus despite the fact that he was also a persona non grata at Hogwarts, all because Severus wasn't able to see the miniscule magical talent Petunia had. Strong enough to register for Hogwarts, but weak enough to be compared to Muggles and squibs.
While Severus spent his time trying to connect to Lily despite the ravine their respective House created in their friendship and dodging the Marauders, Petunia spent her time steaming in jealousy over Lily. Petunia showed interest in a young James Potter since the time he first bullied Severus in the corridor, but he only had eyes for Lily. Petunia spent the whole summer family vacation in France trying to avert his eyes from Lily. Thus she spent the rest of her time in Hogwarts with this mission, to dethrone pretty and popular Lily from his eyes, her fellow students' eyes, the professors' eyes and her parents' eyes.
The whole school soon found out about the love square between Petunia, James, Lily and Severus in their fifth year. However, meddling Albus Dumbledore didn't believe in Gryffindor and Slytherin alliances let alone relationships that might lead to marriage. So, he left Sirius Black and James Potter with a book about advanced conditional wards. It was poetic justice that Petunia and Severus ended up bonded together after that kiss being trapped in that classroom for a week. The Prophet had news flurry with a permanently pregnant virgin Petunia Evans.
Finally, Petunia was more popular than her sister.