(CW: reptile, humiliation, very graphic merge and anatomy changes
Feel free to continue this as you like, I’m just writing the general ideas of the branch)
The evil sister smiled, immediately knowing what she would do now. "Sit on the floor" she ordered Jeff pointing the ground and he just followed her words.
"Now your legs will fuse into a single limb to look like a single pointy tail, after they are fused they will get extra mass to make them longer and thicker, adapting your bones and muscles to the change" she sat with her evil grin waiting for Jeff's changes.
Jeff's transformation was painful and definitely not quick, his legs closed to be together and melt into a single limb like if they were made of clay, the bones and muscles combined together too to form a single long set, at the tip of the limb or rather "skin tail" was a pointy end as Lilith wanted, not even a single foot or a "leg", but rather a 5 feet long tail below his waist, looking his new long limb extend through the floor.
"I know what you must be thinking", Lilith giggled looking how funny her brother's new form was, "that I am a bitch, that I am crazy... But I just did this to be closer as siblings!" Jeff didn't see a single good intention behind Lilith's expressions. "You see, I've always been fascinated by mythical creatures, specifically nagas, a half-woman and half-snake powerful hybrids with special powers to lure their preys" she explained as Jeff looked at her horrorized, thinking that maybe she wanted to transform him into one of those things. "Oh and no, you're not becoming a naga... I'M the one becoming a naga! You just did the hard part for me!" she said with a mischievous grin.
She wanted to become a naga but transformed him? Jeff tried to connect the dots and realized that things were worse than what he thought. "remember what I said before about becoming closer? Well I'll be the human upper part of my naga body and guess WHO will be my tail!" she laughed seeing Jeff's expression covered in fear.
Seconds later Lilith began to take her clothes off, it didn't take long as she was only wearing an oversized hoodie and some panties, not even pants or a bra. After getting fully naked she slowly walked around Jeff's seated tailed body, static and unable to speak to complain. "I can't wait for you to become my tail, moving and sliding against your will, drowned in my full control" she chuckled before standing in front of Jeff but turning her back on him, his head was almost at the same height as her ass.
"The connection will be made through my ass, so let's get started!" Lilith bit her lips while she looked back spreading her anus with both of her hands, "my ass hole will be able to take your head and make it fit normally" she bit her lip harder as she approached her butt closer to Jeff's head. Meanwhile, Jeff's eyes got wider seeing her sister's rear hole expanding in front of him, touching it with the tip of his nose. Lilith used her hand to push Jeff's face deeper until everything from his neck up was inside, she let a big and loud moan escape as she was penetrated by her twin's entire head, her anal muscles closing around to keep him harder.
In the current state Jeff looked like a long skin log coming out from Lilith's butt but it was actually the total opposite, for Lilith the sensation was so amazing that she let herself enjoy her moment for a some extra seconds, the only thing bothering her was that Jeff's hair tickled a little inside her. Jeff felt in hell, tasting and smelling her rancid uncleaned insides, he would try to push himself out of his situation but her anal muscles were too strong and he couldn't even move his arms. "Now your arms will dettach from you and slide to MY torso below my existing pair" she said smiling while moaning, Jeff felt horrible, like if they were ripping off his arms, his bones and muscles drastically separated and slid up, passing through Lilith's waist and stopping at her torso, reconnecting with her torso and making themselves part of her, leaving Lilith as a four-armed woman. "We can proceed with the next big phase!" she regained her posture using her new pair of arms to grab her own boobs while she crossed her arms with the other pair, "my legs will merge with your chest and the rest of your torso as my ass cheeks fuse with your back, fully making you my tail below my waist. Our nerves will combine in order to let me feel everything from my new tail which I will know immediately know how to control and move with it, your bones will become really flexible to let me move as I like. Speaking of bones, my tailbone will connect with your skull and the rest of your spine" she said still grinning, watching fascinated how her body fully assimilated Jeff... or what was left of him attached to her ass, there was longer a trace of him as his flesh merged with Lilith's body from her waist up, looking like her almost 7 feet long skin tail due the mass she absorbed from him.
Lilith inspected her body, moving by dragging her tail on the floor like a real snake would do, she did it like if she has always been like that. She stopped in front of a full-lenght mirror, she was more than happy with her current changes as a four-armed woman with a skin tail and a vagina in the middle of her upper body's crotch, but she obviously wasn't done. She still felt some tickles with Jeff's hair and head still inside her despite them being totally merged from the outside, she still had some internal changes to do by she couldn't be more eager to do it. "I think you can't hear me anymore since your head is fully inside me, but not that it matters" Lilith giggled, "your hair, ears, eyes and nose will disappear from your head" Jeff couldn't tell what was going on as everything around him was just Lilith's bowels a their horrible oddor, things got worse as his eyes got shut covered by his own skin, the same thing happened to his ears and his nose and hair retracted to his own head, just leaving his mouth. "I want your head fixated in there, so your teeth will also vanish and your mouth will connect with my ass hole so that my waste goes through your body, the rest of your head will merge with my anal muscles. You will be able to taste and smell everything that moves through it. Then your anus will be MY anus at the end of MY tail" she laughed feeling the merging again, touching her boobs and her pussy, doing it with different arms every time.
Jeff just screamed inside, not knowing whats happening, but nothing was heard outside. Finally his head stopped moving due both his loss of control and the fixation it got. He felt his teeth begin pulled back into their gums and than his lips connected with the rest of Lilith's "shit-tube” in front of it as if he wanted to kiss it. He mentally screamed in disgust, but was unable to do anything anymore. He could still feel his former body and its changes.
"Much better, we're truly one now" She smiled feeling their internal changes finish with Jeff as the extension of her bowels. "We still have the final changes left. My tail will be fully covered by red scales, from the tip of it to below my belly, but the underside of my taill will be a path of white scales" Lilith saw with pride how her skin was replaced by bright white and red reptilian scales that adorned everything below her belly, while Jeff felt like ants crawling over his body, probably even worse.
Lilith was essentially finished, looking like a true naga from the myth, but she still wanted to give herself some special details. "Now my four hands and forearms will be covered with red scales too, replacing my nails by black strong claws" Lilith felt again the wonderful sensation in one of her pairs of arms, Jeff felt it too in the arms that were previously his. "My eyes will turn yellow with vectical pupils like a real snake predator and I will be able to hypnotize people with them any time I want. Finally my mouth will gain the ability to unhinge and open even wider and two of my teeth will grow into fangs" her wish came true and her facial and mouth changes appeared as she said, unlike Jeff, for her the transformation felt good and even a little pleasurable.
"Much better!" the naga woman said looking herself in the mirror again to see her fully monstruous appearance, she noticed one thing, a long reptile penis came out from her vagina like coming out of its sheath, it seemed that she absorbed Jeff's penis and transferred to her becoming a herm, she liked the addition and wouldn't complain. She pushed the cock by its tip back to the insides of her pussy, capable of switching between her male and female parts.
Jeff was mentally whimpering in pain, while Lilith enjoyed every single changes. To him they were really painful, as he could feel every bone cracking and changing inside. He nearly blacked out when his limbs merged and his mouth was fused with her butt hole, but he was unable to do so. He was reduced to an extension of Lilith body that could only feel and think, the later at least when not in pain. He didn’t know that Lilith had let him keep his senses of smell and taste, too, but he would soon find out and hate it.