(CW: humiliation, very graphic merge and anatomy changes.
I’m just doing the main ideas of the branch, feel free to continue the story as you like)
Lilith suddenly got a really good idea to not only get rid of Jeff but also give him the punishment he deserved. "First, take off all your clothes and throw them away, you won't need them anymore" she grinned controlling her brother.
Jeff's body movement completely escaped his control, doing what his twin just said and remove every single piece of clothing until he was fully naked, his face turning red as a tomato fully shamed of what he was doing even if he didnt want to.
“Now your arms and legs will be swallowed by your torso, your cock will disappear too, and your anus will close, leaving you as a limbless body with a head.” she chuckled waiting for his changes.
Jeff’s body obeyed against his will, his body began to sink inside himself, bones and muscles went inside his own torso like something falling in quicksand. As he “devoured” his own arms and legs those extremities dissolved once they were fully inside Jeff, his penis and testicles shrunk until there was nothing more leaving a smooth skin surface on his crotch, finally his butt hole was covered by his own skin in order to erase his anus. He transformed into a single torso with a head as Lilith predicted, making him impossible to stand up and lay around the floor.
Lilith smiled at her tortured brother struggling to move on the floor, crying and probably would be begging for help if she allowed him to talk. “Nice look bro, but we still have some changes left. You now need to shrink enough to hold you with my hand.” As she said Jeff’s size was drastically reduced until he had approximately the size of a flashlight. “Good job” she clapped continuing to make fun of him. Lilith lowered her body just to pick Jeff’s small self, she thought he looked kinda cute in that way, small, unable to talk and without his limbs looking like a goofy human toy. The female twin sat on her bed, removing her panties as she was only wearing that and an oversized black hoodie, just to spread her legs. "I want a long mirror to appear in front of me" as she wished a mirror popped up from nowhere, being able for Jeff and Lilith to see her bushy, uncleaned vagina in the reflection.
She had a happy expression looking at Jeff's tortured body on her palm. "I guess you're still clueless about your fate, so let me be very direct with it... I will transform you into my pussy!" as she said the last sentence Jeff's face turned into a look of total hoplessness and fear, not believing what his sister had in her mind. "I think that wasn't clear enough, so let's get started!" she kept her grin taking her limbless doll-sized brother and beginning to penetrate her vagina with him until he was fully inside, having just half of his head outside of her folds.
"Now you will begin to merge with my insides. first your throat will become my vaginal canal, making your stomach my womb." Lilith bit her lips beginning to feel the pleasure as Jeff felt like being in hell, his humanity was being erased as almost every single part of his anatomy was forcefully changed, his stomach replaced his sister's womb and turned to be bigger than her original one, having his mouth and throat as the connection of it, making his stomach closed as the rest of his intestinal track vanished.
Lilith could feel her new vaginal track forming, now was time for the second change. "your lungs will fuse into a single organ, my bladder, then they will connect with your nose that will be my urethra so your nostrils will merge into a single hole." Jeff felt like his lungs were smashed together, feeling how they began to fill themselves with Lilith's fluids, he no longer needed to breathe with them as she changed that aspect of himself to assume his role as a vagina. Jeff would scream if he could, the horrible sensation of his organs rearranging, merging and morphing themselves was horrible, but Lilith couldn't care less, smiling with lust looking at his pain expression in the mirror.
"I will not get rid of you completely, that would be soooo easy... instead I'm planning you to never find comfort in the rest of your days" she let out an evil laugh, "you will never rise above my belly so i think that makes me the big sister!" she kept laughing while she could feel and sense Jeff's struggling with his transformation, but that just ended up pleasuring her even more. "Imagine me making all your friends "play" with you, I could make your girlfriend give you a couple of kisses down there! Or even better, perhaps I could make you two closer by turning her into my ass hole! Posibilites are endless."
"I have done many internal changes to you, so let's change your appearance a little. Your mouth will turn into a vertical slit that will take the function and look of my vaginal lips, sucking your teeth away from your mouth then your ears will disappear while the rest of your nose turns into my clit" Lilith spoke, feeling immensely powerful having control over her annoying brother's form, she even began to get wet by her high sense of authority, and Jeff could definitely feel that. The face of the soon-to-be pussy boy changed as his sister said, his mouth rotated some degrees until looking like a vertical opening, his teeth were swallowed by his own gums and it hurt like hell, wanting to cry again, all that while his nose no longer looked like a nose but as Lilith's pleasure button instead. His ears also disappeared and sealed the holes of the sides of his head (if that could still be called head to him), but dispite that he could still hear from Lilith's ears.
"Oh my..." Lilith bit her lips again now rubbing her clit formerly Jeff's nose while grabbing one of her boobs with the other hand under the hoodie, "you feel hundreds of times better than my previous pussy, I think you're an excellent candidate" she giggled but then she couldn't concentrate in her pleasure section as she saw Jeff's crying eyes above her vagina. "Right, I still have a change more for you! your eyes will be sucked inside my crotch and become my ovaries, but don't worry, your brain will still be intact so you can taste, feel, hear and smell" she joked about Jeff's terrible situation, "for the final touch your hair will replace my pubes". Jeff looked how his eyes rolled then suddenly everything turned black, actually his optic nerves were disconnected from him and reattached to Lilith's womb as her fallopian tubes while his eyes fully turned pinky and fleshy with their role as ovaries, finally if that wasn't enough he felt how his scalp was assimilated by her crotch to become Lilith's pubic hair.
Lilith couldn't hold it anymore and quickly shoved one, two and three fingers inside her, she not only felt an increase in pleassure but also felt how spacious it was inside her. She kept masturbating while Jeff felt every, single, thing. All of his nerves were the property of Lilith's crotch. After a couple of minutes he couldn't hold it anymore, Jeff vomited through his "mouth" in order to shoot his sister's vaginal juices.
Lilith looked at the mirror with a mischievus smile, stained by her feminine fluids. She looked down and slapped her crotch, "you did it very well Jeff... no, you no longer deserve that name, I should call you just pussy from now on, that fits better" she got out from her bed to stand up, looking and feeling the wonderful change she just did to her brother, she could be considered a single child now but she still wanted to play with some ideas. Jeff wanted to scream at the top of his lungs (or bladder?) but instead he ended up dropping the last bits of Lilith's cum. "don't worry pussy, I'm not going to make everyone forget about "Jeff", so it will be funny to see everyone's reaction after your disappearance... or after I reveal them the truth hehe."
Lilith walked a little forward to grab her panties and wear tjem again. “I hope you get used to this, because I’ll never shave or clean you. You've always been a dirty pussy so nothing has changed” Lilith mischievously grinned patting her crotch as Jeff felt every small slap. "from now on your only tasks are to make me feel good, taste my piss and my monthly period, let's see how long you can endure that, not like you have an alternate option" Jeff heard Lilith's evil laugh again like she wasn't getting tired of that. He couldn't believe it, he was trapped, reduced to his twin's sexual organ for the foreseeable future, he prayed for something to change but at this point it was just impossible, Lilith was about to rule the world and he was just her pussy.