Sometimes Lily and Petunia were sisters and sometimes they weren't.
When Lily is born she and Petunia are sisters. Petunia is ecstatic to be an older sister, and cannot wait until Lily is old enough to play with her.
When Lily was two Petunia started her first day of school, and Lily always followed Petunia around and wanted to do absolutely everything that she did and therefore also wanted to go to school. Petunia felt bad that Lily could not join her for her first day of school and told Lily that when she got home from school that they would make up their own school and they would play all night.
They were not only sisters but they were best friends. When Lily became a little older around four or five she was allowed to go to the park down the road with Petunia. Lily loved the outdoors, she loved climbing trees and she loved going to the park, and she especially enjoyed swinging. Around this time that is when Lily noticed that she was a little bit different from Petunia and that when she was happy or sad or angry she could make things happen. At first, she didn't think anything of it and carried on playing as any other child did and didn't think to even mention it to Petunia. But as Lily got older the 'strange' things that she could do drew in Petunias attention. Unlike Lily who is not bothered by the fact she was a little different, Petunia was appalled that her sister was able to do these things. However, she was still her sister and so Petunia thought that perhaps it was a phase in that Lily would grow out of it eventually.
The only problem was Lily did not grow out of it and when she was 9 she met that awful boy named Severus, who could do similar things to what Lily could do. The boy was odd that begin with always wear long sleeves and always looked underfed. Petunia didn't like him, she thought he was a bad influence on her sister encouraging her to do those strange and awful things. In one day she got fed up watching Lily and that boy doing those strange things and told Lily that she was going to tell their mother that Lily was doing bad things so that she would not be able to go to the park anymore if Lily didn't stop. Because Petunia was her older sister that was her job to protect Lily from bad influences. But that Snape boy turned around and told Petunia that really she was not doing anything wrong and that when Lily was 11 she was going to get a letter from some school called Hogwarts where she would learn how to control her magic. Yes, that nasty Snape boy called Lily strange abilities Magic. Like that meant something and it was supposed to be awed by all.
And sure enough on Lily's 11th birthday a little over two years after that conversation in the park, a letter came for Lily announcing that she had been accepted into Hogwarts School of Witchcraft and Wizardry. Petunias mother and father were ecstatic that the letter came that meant that there was nothing wrong with their little girl and that she was special. Perhaps in the best way instead of a bad way. Petunia disagreed though she felt as though her sister was special in the worst kind of way in which magical things were such preposterous ideas. However, sure enough, a stern looking woman named Minerva McGonagall who claimed to be the deputy headmistress of Hogwarts showed up later that day to explain to Lily and her parents all of the things I need to know about Hogwarts and The Wizarding World. and after that, she took them to Diagon Alley where she explained how it works and explain how Lily could get her things for school.
If Petunia was being honest with herself she was in awe of The Wizarding World. There were so many amazing things to see and do and after watching her sister get her school supplies her cauldron her wand and her fancy spellbooks there was a part of Petunia that wanted to go to Hogwarts too. And since the Deputy Headmistress, Professor McGonagall had told them that Professor Dumbledore was the Headmaster of Hogwarts she decided that she was going to write a letter to him asking if she could go to to be closer to her sister.
Petunia's letter was very long and practically begging Dumbledore to let her go to Hogwarts and Dumbledore had actually written back in a very timely manner and told her in the kindest way that it wasn't possible for Petunia to attend Hogwarts because she was not capable of doing magic and that she wouldn't be able to participate in the lessons which essentially meant that there was no point of her attending Hogwarts at all. This upset Petunia greatly that this school and magic was taking her sister and her best friend away from her.
And so when Lily left platform nine and three-quarters on the train to Hogwarts, Petunia had decided that she was right when she thought that Lily was a freak and an abomination. She wanted absolutely nothing to do with Lily or her freak magic or her freak School. As far as Petunia was concerned Lily wasn't her sister anymore.
For years petunia that Lily was nothing more than a freak. And when Petunia met Vernon they hit it off almost right away. At first, she had told him that she didn't even have a sister that she was an only child. But as time went on and the relationship got more serious she decided to tell him that she did have a sister but she was away at a school for badly behaved adolescence. However once again the story changed when Vernon had asked Petunia to marry him. Of course, she said yes because he was everything a man should be. Successful, wealthy, and only had eyes for Petunia. But when he had asked her to marry him she was able to tell him that her sister went to a freak school for kids that could do magic but that he couldn't tell anyone and if anyone asked that he was to tell them that she went to school for badly behaved adolescence. But over Christmas holiday in 1977, Petunia and Lily's mother had practically begged Petunia to go out to dinner with Lily and Lily's new boyfriend so that they could all get to know each other and Petunia and Lily could talk about Petunia's wedding. Petunia reluctantly agreed.
And how wrong Petunia was to agree, the dinner was disastrous. Lily's boyfriend offended her over and over again. He felt the need to be amused by Vernon the entire time and that awful James Potter had the audacity to say that he didn't even own a car but a racing broom. And when Vernon voiced that he assumed that Wizards lived off of unemployment benefits, Lily's good-for-nothing boyfriend turned around and said he was a very wealthy wizard with a lot of gold in a bank called Gringotts. That was the final straw for Petunia and Vernon so they left the restaurant angrily. Lily head burst into tears, and James who have felt sorry promised that he would make it up to Vernon and Petunia if the chance ever came up. But as far as Petunia was concerned this was another reason why she and Lily were not sisters.
When it came time for Petunia and Vernon's wedding, Lily was of course not a bridesmaid. Petunia had not wanted her as a bridesmaid before that fateful incident at the restaurant but most certainly Lily was not allowed to be a bridesmaid after what happened. Not to mention how Petunia was tired of being overshadowed by Lily and how perfect Lily was to her parents and everyone else. What was the point of Lily being a bridesmaid or even the maid of honor when Lily wasn't her sister or even her friend.
In February of 1978, barely a month after Petunias wedding, she and Lily's parents were murdered. Neighbors claimed that there was a glowing green skull in the sky above the house shortly before their parents were found. When Petunia found out that it was some wizard that killed her parents she blamed Lily for going to that good-for-nothing school. Once their parents had the lowered into the ground, Petunia went up to Lily and told her just what she thought about how Lily wasn't her sister, she wasn't her friend, she was just a freak.
In November 1978, James and Lily got married. Lily showed up at Petunia's door at number four Privet Drive a month prior to her wedding and asked Petunia if she would like to be a bridesmaid in her wedding and personally handed her a wedding invitation. Petunia rudely declined claiming she wouldn't want to be anywhere near there are a whole bunch of freaks lurking about. And after all Petunia pointed out she and Lily weren't even sisters.
The last time she heard from Lily was when Petunia received a birth announcement announcing the birth of Lily's son Harry James Potter who was born on July 31st, 1980. But as far as Petunia was concerned she didn't even have a sister and she put the birth announcement right into the bin. Petunia didn't even bother to give it a second glance.
But on November 1st Petunia opened up her door to find 15-month-old Harry James Potter on her doorstep with the letter that said that her sister had died protecting him and that there was no one left to raise Harry but her. And Petunia looks down and realize that the only thing she had left of her sister was this little boy with green eyes just like Lily's. and for the first time in years, she thought of Lily as her sister. She couldn't stand that little boy in her arms who look just like his father but had her sister's eyes and was the reason that her sister was dead. And from that day forth Petunia decided even though she would take care of her sister's child he would know for certain that he was not truly welcome in her house or truly a part of her family because he is who took away her little sister. Because at the end of the day she knew that whether she liked it or not, Petunia did always have a sister.