I do apologise for not giving you the smut you requested. I couldn't get it to work. :|
But without further ado, the rest of your request: Ginny/Cho and a Weasley jumper.
She'd never tell her mother so, but Ginny Weasley hated her Christmas jumpers. Through the years, she'd managed to get rid of them, one way or another. This year, she was stuck.
Standing at the mirror in the prefect's bathroom, Ginny held the sweater up. The puce colour clashed horribly with her red hair and she sighed in dismay. Behind her, the door opened, allowing Cho Chang entry. Ginny turned, smiling at the older girl.
"Thought you weren't coming," she said, moving towards the already-filled bath. Cho followed her, a predatory gleam in her eyes. Ginny stepped into the water, still clutching her mother's gift. Cho stripped and joined her, pulling the redhead close.
"Lose the jumper, Weasley. It's not your colour," Cho murmured. Ginny dropped it into the water, gasping as Cho's hands moved across her body.
"Mum'll be upset at me," Ginny said an hour later, apropos of nothing. Cho, lying next to her, propped herself on an elbow, trailing her other hand over Ginny's stomach and between her thighs.
"Why's that?"
Ginny whimpered, squirming under Cho's skilled hands, then replied: "I lost my jumper in the bath."
"Accio jumper," Cho commanded, and the sodden fabric flew out of the bath. She muttered a drying spell at it, then held up the results. The once too large sweater was now too small for a House-Elf.
"Guess that takes care of that, then," Ginny said with a laugh. "How can I ever repay you?"