The Joys of Twincest.
"I don't care," Molly Weasley scolded her sons. "Brothers just don't do that kind of thing together. It's wrong."
"But Mum, we're identical twins!"
"Practically clones of each other."
"When we suck each other off,"
"it's the same as auto-fellatio!"
No sooner had the last word left Fred's mouth than Molly fainted dead away.