The two sisters weren't able to relax for very long before Flora's mutation began. She felt a tingling on her forehead before her vision was suddenly expanded by the addition of six new eyes. Her canine teeth then pushed out into a pair inch-long fangs which began to drip a clear liquid. When she went the feels the new additions to her face, she found that her hands were becoming covered in black chitin. She watched, all eight eyes wide, as the chitin spread up her arms to just above her elbows. A short flare of pain lanced through her forearms as they took on a form more suited to the exoskeleton that now covered them, with external joints protected by protruding plates of chitin.
Sofia was unable to see her sister transform since her cock-tongue was still knotted in Flora's vagina, only able to tell what was happening by the confused gasps coming from above her. She was suddenly yanked upwards when Flora stood up. Her body was then lifted upwards between Flora's legs, before her entire body below the neck twisted completely around, leaving the front of her torso pointing towards the ground. Her head and neck completely fused with Flora's hips, taking on a form resembling the head of a spider with Sophia's face attached to the front of her sisters crotch, her mouth taking the place of Flora's vagina.
“Sofia, can you still talk?” Flora asked staring down at her sisters face, her canine cock-tongue and ballsack almost looking like they were in the proper position on Flora's body, if she didn't have a vagina originally that is.
“Yeah, it feels really weird but I'm still here” Sofia replied, her cock-tongue making her words come out muffled.
A wave of relief washed over Flora, the mutation liquid merging people together was was rare, but it was documented that sometimes one or more of the people involved in the merge lacked any way to communicate. So long as her sister could still talk, Flora was certain that they could make it through this, Sophia might even be able to still take her finals if everything worked out.
As if responding to her relief, the changes to the sisters fused body continued. Sofia's limbs morphed into eight pointy, chitinous, spider legs and migrated to the sides of her spider head. Her now limbless body bloated into a giant spider abdomen, large enough to hang off the other side of Margret's bed. The six breasts still on the underside of the sisters abdomen expanded to the size of basketballs and began staining the sheets of the bed with milk. The cock and balls that were originally between Sofia's legs, but now sat at the end of the sisters abdomen, swelled larger as well. The shaft grew enough to touch the floor and then some, over three feet long and as thick as a one litre bottle, while the balls became as large as the breasts adorning the abdomen.
As the changes to the sisters abdomen finished the front end of the two's body began it's own set of changes. Flora's breasts exploded in size, quickly becoming so large they nearly covered her belly button. Her nipples then pulled inwards and became a pair of vaginas, returning the sisters original anatomy in the completely wrong place. Fiona's legs, now not even long enough to touch the floor, were transformed into a pair of pedipalps that Sofia was on sole control of, granting her a small amount of ability to manipulate her surroundings. She soon found a use for them as her cock-tongue began to grow, allowing her to stroke it with her chitinous appendages. Her canine knot filled her mouth as the shaft reached a final size of a foot and a half long. Her balls had also grown, now the size of coconuts hanging heavy with sperm below her face. As Sophia continued to stroke her phallic tongue, Flora had begun to finger her nipple-vaginas and both sisters worked in unison the rub their six milky breasts and their massive back cock on the bed.
All the stimulation quickly brought the sisters to orgasm, both of their cocks blasting out enough cum to almost completely undo their work cleaning Gemma's mess. Their orgasm lasted for nearly a minute before both their cocks softened, Sophia's cock-tongue shrinking enough to fit inside her mouth. It was only then that both women realized that someone had opened the door, directly in the line of fire of their cum.