"Oh, hey mom," Jeff said casually, synthetic cum dripping from the fleshlight in his hand. He had opened the door to her, and as he'd hoped, the look on her face was priceless.
"... Jeff? Is that you? What happened--?"
"Good timing! If I keep using her dry like this, Chelsea's going to break in no time." He grinned.
"What? Chelsea? What do you..." Jeff stepped forward, and mom tried to take a step away, but found herself stuck in place.
"I guess the quickest way to explain is just to do this." He touched his immobile mother's head with the Chelsea fleshlight, and she began to sob as her daughter's recent memories and her emotions flooded into her head. Trapped, humiliated, still in physical pain from her rape.
"How could you," his mother choked out through her sobs.
"She wouldn't see how important she was to me, so now we can be together without her confusion getting in the way! I should have known you'd take her side. Luckily, with what I have in mind you'll be helping her out, with the extra bonus that you'll have some time to think about jumping to judge me." With a gesture, Jeff started the first part of her transformation.
"Ah!" Mom grunted with pain and doubled over as her vagina suddenly spasmed painfully, ejecting a gob of warm fluid. Able to move her arms, she reached down her pants to see what it was, terrified she would find her insides spilling out or something. Instead, when her fingers came back out they were covered in a clear, slick gel. "What is this?"
"Like I said, Chelsea's going to split in half after a couple more uses if I don't make using her a little gentler. So I'll keep you close at hand!" He gestured again, and his mother's clothing vanished as she began to float through the air. She was carried by an invisible force to the wall just next to Jeff's bed, her legs folding up uncomfortably against her body.
"Stop! What are you doing, Jeff, please! Stop!" Her head started to sink into the wall, quickly taking in her eyes, then nose, then mouth; she struggled as much as the force keeping her doubled over would allow. She wasn't merely going through the wall, of course: she was being fused to it, the extra bulk of her body being erased as she was pushed in. Mom was still conscious, but was losing all sensation except for the parts of the body that were outside the wall. It wasn't long before her feet and legs, her entire torso sank away. It stopped when she was just a pussy and part of her buttocks sticking out of the wall. Jeff found her anus distasteful, and smoothed it away, but kept the protrusion made of flesh instead of something artificial.
Jeff walked up and put one hand under his mom's vulva, and pinched her clit firmly with the other. It twitched and spurted out a gob of clear lubricant gel, which Jeff knew was a painful spasm for his mother. He pinched five or six more times, each time getting a full gob of lubricant, which quickly overflowed his palm. "See? You don't run out! I can get as much as I'll ever need out of you." He began to smear the lubricant on his mechanical penis. "Oh, and I know you like being able to keep an eye on your children, so..." He tapped Chelsea against his mom's vulva. "There, now you'll each be able to feel each other's emotions and sensations. I've brought you closer than you'd ever be without me." He plunged his cock into Chelsea again with a loud, slick squelch.
Mom, of course, couldn't hear any of that last part. All she knew is after sinking into the wall and a sensation like someone wringing out her vagina half a dozen times, she could suddenly sense Chelsea. There was first pain and sadness, then a moment of confusion as Chelsea realized she could feel someone else. That moment ended when mom felt Jeff's cock thrust into Chelsea, and then she was back to pain and humiliation and disgust. Mom tried to do something, to escape and go help her baby, but twitching her pussy was the only mobility she had left. All mother and daughter could do was endure their rape together.
When he was done, Jeff listened in on the feelings being exchanged between them, and felt them comforting one another after the ordeal. They'd always have each other, he realized. And that wouldn't do, not for Chelsea. This was her punishment after all. So he changed one last setting on his sister's transformation: like flicking a switch, Chelsea could no longer love, or feel love from, her mother. Emotionally speaking, Chelsea was trapped in this state with a complete stranger. Mom could feel the change, too, and after a few days she realized this wasn't temporary, and that her daughter could no longer love her or take comfort in her. And this was how they were left; Jeff never did change them back.