Disclaimer: JKR owns it all, I'm just playing.
She didn't like it when Lily did weird things. She shouldn't be jumping off the swings like she does. She was going to get hurt. Then Mummy would probably say it was HER fault. Lily never did ANYTHING wrong.
She had that weird friend too. He was a little scary, and he didn't look like any of the other children that the girls played with. He didn't want to play with anyone but Lily anyway. He was taking her sister away from her.
Then Lily got that letter, from that weird school. That boy did too. She was going to go away, and it would never be the same. Lily was leaving her alone.
She waited for school holidays, waited for Lily to come home. She listened to stories about how wonderful that school was. She was left out again. She missed her sister. Even when Lily was home, she was too busy to spend time with HER. She was too busy doing homework and writing to her friends.
She realized that it would never be the same again, and it felt like someone put a knife through her heart.
A/N: thank you to chocolaterox92 for this idea.