Split

Jon gasped when the flash hit him, and he crumpled to the floor. Jonnie felt dizzy, and shook her head to clear it.
On the floor, the naked Jon, suddenly free of the girl body, looked down at his original boy equipment, his slightly hairy chest, with both joy and disappointment.
"That was weird," said Jonnie, getting up to sit on the bed. She looked down at the dress, then at Sandra, and her naked brother Jon over on the floor. Jon covered himself, suddenly very self-conscious.
"I'm naked... I didn't think it would leave me naked!" said Jon.
"Ew! Boy parts!" said Jonnie.
"I agree," said Sandra, happy her wish worked out for everyone. "I wish Jon were fully clothed in his normal attire."
Jon felt his boxers appear, then his jeans, and his tee shirt. He felt relieved.
"Thanks, Sandra," he said. "Now... explain this twin sister bit..."
"Yeah, I'm kinda confused," said Jonnie, "I mean... I know what just happened, but I remember always being a girl... and a cheerleader... and a ballerina... and a lesbian."
"Modified conditions," said Sandra, "I just opened up a new possibility for interpretation. Sorry about the sudden sibling, but this way I didn't need to invent a new identity for my new lover here."
Jonnie giggled.