VOCALOID Forever

A pair of 14-year-old fraternal twins, Ashley and Caiden Manderstein, accidentally use their strange powers to bring VOCALOIDS Rin and Len Kagamine into the real world. Things quickly spiral out of control for all four kids. Ashley, a big fan of the VOCALOIDS, is immediately overjoyed, while Caiden seems to see everything as a giant nerd-fest.

A fanfic by Ashley of the Sound, it remakes an earlier story, Twins Times Two. Found here (archived)

According to the author, the designs for Ash and Caiden came long before she had ever heard of VOCALOIDS, and the resemblance between Ash and Caiden is totally coincidental.

Characters:

Ashley Manderstein, a 14-year-old girl with neck-length blonde hair and blue-gray eyes. She is Caiden's twin sister. Shown as hot-headed, comedic, and sharing a strong relationship with her brother, she was a big VOCALOID fan from the start. She has sound-based powers whose full potential have not been explained yet.
Caiden Manderstein, 14-year-old boy with short blonde hair and blue-gray eyes. He is Ash's twin brother. He is shown as perverted yet sweet, and took a liking toward Rin. He is revealed to shift into a strange cat-like creature at night because of genetic mutation at a summer camp. His powers are electric-based and because he shot his sister's computer with electricity, he accidentally brought Rin and Len into the real world. He thought VOCALOIDS were stupid from when his sister showed him a VOCALOID video.
Len Kagamine, a 14-year-old VOCALOID boy with neck-length blonde hair and blue eyes. He is Rin's twin brother. He is shown as smart and quiet and nervous to speak, with flat emotions.
Rin Kagamine, a 14-year-old VOCALOID girl with neck-length blonde hair and blue eyes. She is Len's twin sister. She is talkative and was happy to beat up Caiden after he sexually harassed her via Miku Miku Dance. She gets along well with Ash and her brother but is yet to soften up to Caiden.

Tropes used in VOCALOID Forever include:

And I Must Scream: Rin and Len, until being released by Caiden, are forced to do whatever their master wishes them to without the ability to say no––and what Caiden does to Rin on MMD before bringing her out is mild compared to some of the things you can find on YouTube.
Arson, Murder, and Jaywalking: Crazy VOCALOID fans force the Kagamines to sing bad things, do horrible things on MMD...and they also write crappy fanfics.
Domestic Abuse: A rare non-romantic example here. Ash and Caiden beat each other up pretty badly, but genuinely love each other.

"We NEVER fistfight. Yeah, he tasers me every one in a while, and I'll blow his ears out, but nothing like this!"

—Ash defending herself after her mother accuses her of punching Caiden in the face.

- It could be considered just plain sibling fights.
Hair of Gold: Ash, Caiden, Rin, and Len.
Half-Identical Twins: As seen above, Rin and Len are semi-identical twins as are Ash and Caiden (or at least Ash and Caiden are presumed to be since they, like the Kagamines, mirror each other's looks).
Petting Zoo People: Caiden in his cat form.
Rage Against the Author: The Kagamines want to get back at every person who ever "abused" them.
Strong Family Resemblance: Rin and Len together and Ash and Caiden together, mainly because they are all semi-identical twins.
Twincest: Averted. Rin and Len highly despise Twincest, though the author claims it's nothing personal but it does reflect her views.