A/N: Though I'm not sure if I have it quite right, this is my take on Sam and Melanie's relationship.
You know Sam hates you, but you don't care. It's sad, really; she's not just your sister, she's your twin, and the strongest emotion you can conjure up for her is apathy. You wish Sam made you mad because that means you would feel something for her.
You used to be best friends. Of course, she would spend her days doing crazy things with Mom (once, they covered every inch of the countertops with frosting, and then licked it off. You wanted to puke) while you sat in your room and read, enjoying the escape. But at the end of the day, you would share with Sam, you would talk. You loved her.
Of course, if you were honest with yourself, you would admit that it was only because she took care of you. Sam would find food when Mom forgot to buy it, Sam would beat up Tommy Huggins because he called you fat, and Sam protected you from any and all unpleasantness. All you had in return was your friendship and affection, and she, mostly because she couldn't get it from anywhere else, ate it up.
Third grade is when it changed. That summer, Mom went on tour with her latest boyfriend, and left you and Sam with the (even for Puckett standards) highly irresponsible J'Mamaw. Sam was Mom again, buying food with money she stole out of J'Mamaw's purse, taking you to the Shays so both of you could shower. School started, J'Mamaw went home, and Mom still wasn't back.
Both of you never, ever let anyone else but Carly know about you mom's absence, not even Spencer. Sam forged Mom's signature and stole food from the Shays and took no prisoners. You were so proud of her, at least until October when Sam was inches away from failing all her classes.
Mom was back by Thanksgiving, but the damage was done; the teachers had conferences and talked with your mom and and with you and with Sam. She was going to fail, even if she got her grades up. Her discipline file was too great, her tardies too frequent, her absences all unexcused...
Sam never looked at the principal on that last meeting- she just stared at you. You felt your stomach doing flip-flops because you knew your brave and proud sister, your Sam who took care of you, needed you now. But you couldn't be there, you knew that if you spoke up in her defense and told the truth that it would mean you couldn't stay with Mom and probably not with Sam. You had to be silent.
Not that Sam would hear it. She didn't talk to you for the next month, and the first words she spoke after her silence were about how she was going to stay at the Shays for a while and to make sure Mom bought food. You went upstairs and cried, and you know Sam only pretended not to hear you.
The next year, Sam was finally in a class with Carly and passed with flying colors. You were constantly studying, often far beyond your grade level; you knew that you wanted to go to Pacific Coast Academy, which accepted nothing less than perfection. In the sixth grade, you made it, achieving something without Sam's help for the first time. PCA was your own, something that you would never share with your sister.
You went away and Sam stayed in Seattle. You never heard from her. Carly sent you an email about iCarly, and you watched it dutifully every week. You sent Carly all your correspondences with Sam; Carly had already told you that all the emails you sent Sam were deleted sight unseen.
You care nothing about Sam, because in the end, she cared nothing about you.