Your name is Jack and you're 19 years old. You live with your mother and your 16 year old sister Lily in a small three bedroom house.
When the Fusion Wave first broke out, nobody could believe it. All day and night, the news stations were ablaze with footage of bizarre and impossible changes as the wave spread out through Asia and to the rest of the world. People were going absolutely mad.
It seemed that the wave would hit your own nation next, and experts still had no idea of what could be done to protect against unwanted fusion. There simply wasn't enough information, and not enough time to gather it.
Most people figured the best plan was to lock themselves inside and stay isolated as the wave passed over. Your own mother subscribed to this plan, so the three of you were all locked in your own rooms, waiting for the inevitable and unavoidable wave.
From your bedroom window, you watched in silence as the sky turned a sickly red. Your hands balled into shaking fists as a barely perceptible wave of energy came rushing towards you, subtly warping its surroundings. You clenched your eyes tightly shut and your entire body was engulfed in an intense heat that threatened to burn you alive. But only for a moment.
In an instant the feeling passed, and you were instead treated to new, equally distressing sensations instead.
You could feel that you definitely still had arms and legs and as far as you could tell, had retained a regular human body shape, but something was definitely wrong. For one thing, you were blinded by some kind of fabric hanging over your eyes, and your limbs felt weirdly displaced, like your arms were on the wrong side of your body or something.
You tried to pull the fabric away from your face, but found that you couldn't move your arms.
This instantly made you panic, and you quickly stood up from the floor. As you did so, you heard Lily yelp in confusion, and you felt your arms windmill in an attempt to maintain balance.
"What's going on? My legs?" Lily whimpered.
Seemingly moving on their own, your hands reached down and groped at your thighs experimentally, then slid up and prodded you right in the... face? Causing you to flinch and clench your toes.
Your whole body jumped at this, and you heard Lily mutter "Oh no..."
Your hands autonomously moved down to the hem of your shirt and lifted it, and suddenly the fabric moved away from your field of vision, freeing your eyes.
What you saw left you stunned.
It seemed that Lily had set up her full-length mirror in anticipation of examining any changes, and now you were in Lily's room, staring at that very same mirror.
You saw your younger sister, with her dark hair and her gothic makeup, lifting the hem of her tank top to reveal a pair of human eyes peeking just over the top of her skirt. Your eyes.
You realized that the sensations you now felt belonged to your sister's own body, which you now shared. You could feel everything she felt, from the weight of the pigtails dangling from her head, to the air passing into her lungs as she breathed.
"Oh no no no this is wrong" she muttered, feverishly unbuttoning her skirt and letting it drop to the ground.
Now your eyes were instead situated above a surprisingly lacy and skimpy thong, which she unhooked with her thumbs and pulled down to her ankles.
With her lower body now nude, you could confirm your suspicions.
Your face was now merged with your younger sister's groin. All that remained of your previous appearance was your two brown eyes situated above your sister's pussy, the short, scruffy patch of pubic hair almost giving the comical appearance of facial hair.
"Is... is that you, Jack?" Lily whimpered.
You attempted to answer her, but your efforts to speak and move your "mouth" only resulted in you flexing your outer pussy lips and squeezing unfamiliar internal muscles, creating a bizarre sensation you had never experienced before.
With a stroke of inspiration, you tried to speak with her mentally instead.
"Yes Lily, it's me"
Her eyes lit up, like she had received your mental message.
"Oh god, I can't believe this." Lily sobbed, "I'm so sorry about this, big bro."
"You have nothing to apologize for, this isn't your fault" you mentally reassured her, but you felt like crying yourself.
You were now permanently affixed to your sister's crotch, able to feel everything she felt, but only able to control her lower body. Your life would never be the same again.