how about this?
Elsa toys with her bound sister, wearing nothing but ice bra and p***, and ice armbinder and legbinder. N*** clamps attached to wires shock her every time she makes a sound. Then Elsa decides to try something new - double selfbondage: chains her legs to sister legs, then inserts double-edged vibrating didlo in both of them, connects their n*** and finally puts her arms in special power-restaining manacles with timer (which moves only when they kiss). Sisters have a lot of fun but later turns out that timer has broken...
whoever gets released first, would be really mad at other and punish her probably) Or maybe Kris finds them)

Anna squirmed, trying to get herself comfortable. She tried to wiggle her foot so the foot clamp didn't dig into her ankle so much, but to no avail. She turned to her sister, who stood over her. "Hey, can you-"

Anna felt a strong jolt rush through her body, coming from the nipple clamps Elsa had put on her. Elsa grinned as she watched her sister recoil in pain. "I'm sorry, were you trying to say something? Please, speak up."

Anna bit her tongue, not wanting to get shocked again. She shivered from the cold ice that kept her pinned to the bed. Not helping matters was the icy underwear Elsa made for her. Try not to make a sound. Try not to make a sound. That was the only thing she could think about. Elsa circled her, desperately trying to get her to talk. "You know, it looks really uncomfortable there. If you want I can let you go, just say the word."

Anna bit down on her tongue even harder. So hard, that it was beginning to go numb. She let up for a bit to give her tongue some rest. Elsa, realizing that she wasn't getting anywhere, decided to change her tactics. "Hey, do remember how ticklish you were as a kid?"

"No, don't-" Anna felt another jolt come from the nipple clamps.

"Let's see if you're still ticklish." Elsa leaned down and scratched the bottom of Anna's foot.

Anna tried her best not to laugh, but she couldn't stop herself. She winced as another shock ran through her body. Elsa continued to tickle Anna's foot, sending shocks through her sister's body. Eventually Anna muscled through the pain. "Stop that! Right now!"

"Fine." Elsa pulled away from her sister. She circled Anna for a bit, until a mischievous smile formed on her face. "You know, this looks like fun. Do you mind if I join?"

Anna just stared at Elsa with a blank look on her face.

"Nod your head for a yes," said Elsa.

Anna nodded her head.

"Good." Elsa tried to position herself on the bed. She released Anna from her shackles temporarily, only so that she could fit on the bed. Soon their legs were intertwined. Elsa put the restraints back on Anna. When this was done, Elsa pulled something out from under the bed. As best as Anna could tell, it was a double-edged dildo. She was correct, as Elsa put the dildo between the two of them, so they'd both be pleasured at once. Anna gasped when the dildo was inserted inside her. She received another shock for this. She looked at Elsa enviously, not having to worry about getting shocked. Anna tried to hold back her moans, but it was all in vain.

Soon she just excepted the pain and allowed herself to enjoy the dildo. Anna moaned as loudly as she could, expecting a massive jolt to hit her. She felt a jolt, but nowhere near as painful as she had expected. She looked at Elsa, realizing she had removed one of the nipple clamps. Anna was about to thank her, but it was premature. Elsa put the clamp on her nipple and began moaning sensuously. This sent shocks not only through her, but through Anna also. So that was her game. Anna responded by making as much noise as possible, and soon they were sending shocks back and forth through each other. Elsa moaned more loudly than Anna, but Anna made more noise overall, not limiting herself to moans of pleasure. This was a skill she had perfected since she was a little girl. She had always been the louder sister. "Hey Elsa..."

Elsa flinched. "Yes Anna."

"Ow," said Anna. "Anyways, what was I going to say? I don't remember. It's all these shocks. They're doing a number on my memory. You know-"

"Please stop!" said Elsa, who was visible shaking.

"No way in hell," said Anna.

"Fine." Elsa took off the nipple clamps, having had enough of this game.

"So I see I won," said Anna. "I guess I've always had better endurance."

"No you haven't," said Elsa.

"Yes I have," said Anna. "Look at what just happened. You gave up."

"No I didn't," said Elsa. "I just thought of something better."

Anna raised an eyebrow. "Oh? And what would that be?"

Elsa rummaged through the chest at the foot of the bed and pulled out a giant pair of manacles. She then began to shackle herself to the bed. "These should dampen my powers."

Anna noticed the ice around her arms and legs start to melt. She broke through the shackles and started to rub her wrists. "Oh thank god."

"Now, these manacles have a unique feature," said Elsa. "They have a timer that determines when they can open."

"How long does it take for them to open?" said Anna.

Elsa grinned. "That's the thing, it depends."

"Depends on what?" said Anna.

"How much you kiss me," said Elsa.

"Excuse me?" said Anna.

"Kiss me," said Elsa. "The timer only counts down when you kiss me."

"Who designed these?" said Anna.

"I did," said Elsa. "I kinda have a kinky side."

"No shit." Anna leaned down and started kissing her sister. "How's that."

"Good," said Elsa. "It's getting there."

"Okay." Anna continued kissing Elsa. She slid her tongue inside Elsa's mouth. It met Elsa's tongue inside, and the two tongues began playing with each other. Elsa's tongue was more aggressive, wrestling with Anna's tongue. Anna briefly pulled her tongue out and Elsa had to promise not to be so rough. Eventually Anna pulled away. "So, how much time's left on the timer?"

"Just a little bit," said Elsa. "Come on, free me baby."

"With pleasure." Anna leaned in to Elsa when the door suddenly opened.

"What's going on here?" It was Kristoff.

"Kirstoff, I can explain." Anna ran to Kristoff, but he brushed her off.

"No, I don't want to hear it." Kristoff rushed out of the room.

Anna followed him. "It's not what it looks like!"

Elsa just lay there, tugging at shackles. "Anna! Anna! How's going to free me?"