A/N - Here you go, as promised, the conclusion to the short story.
It was now late in the evening. Tsukasa was lying down in her bed unable to sleep, which was very unusual for the girl. She was still thinking about Miyuki, and she was thinking about why she was thinking about her so much, it was a seemingly endless cycle and consequently she was distracted for the entire evening beforehand. This didn't go unnoticed by the rest of her family, with Kagami in particular becoming more concerned with her twin. Tsukasa shrugged off her sister's questions and went to bed early, but several hours later, she was still struggling to fall asleep.
The image of a half naked Miyuki stuck in her mind and her perverted thoughts began to affect her physically. Her heart began to beat faster, and she felt aroused. As she imagined the pink haired girls bare breasts, she slowly moved one of her hands onto her own chest, feeling her hard nipples against her nightgown. She began to squeeze one of her breasts gently, letting out a small moan as she did so, before rubbing and kneading it with a little more force. A second hand slid under her gown, groping her other breast directly. She pulled and pinched her own nipple harshly; enjoying the stronger waves of pleasure it gave her. Tsukasa's mind raced, she imagined that she was now with Miyuki, playing with her ample breasts, making the girl with glasses cry out in ecstasy, begging the younger twin for more.
Masturbation was a new experience for Tsukasa, and yet it felt so natural to her. She never doubted her actions, she knew exactly what her body wanted, and she moved on without hesitation. She could feel her pyjamas were now becoming damp with sweat, and the crotch of her pyjama bottoms was now clinging to her intimate area. She clamped her legs together, rubbing her thighs and contracting her groin muscles rhythmically, the feeling was incredible. Her core was heating up even more and her bottoms rode up, digging into her groin, the friction bringing another gasp of pleasure from the girl. She began to moan a little louder and more regularly, she could no longer resist moving one of her hands lower. Her hand slid past her waistband, straight to her wet lower lips. Her fingers rubbed up and down, parting the mound and revealing her clitoris. She ran her fingers over it in a circular motion, her voice now climbing in pitch, it was becoming very loud. She buried her face in her pillows, her face bright red as she imagined Miyuki was the one pleasuring her, rubbing her tongue over Tsukasa's swollen lips.
The purple haired girl was now rubbing her entrance, teasing out more and more fluid. Her bed sheets were becoming drenched and the musky smell was all too apparent to her. This turned her on even more, and she began to feel a strange feeling building up. As she rubbed harder, her muscles began to clench, and her butt lifted off of the bed, her back arching. Tsukasa came hard, and it lasted for many seconds, shocking the girl who never really imagined any feeling like this before. She felt her warm cum flowing down her bum, through her gown onto the bed. As she relaxed back down, she sighed, going over in her mind what just happened. She had just masturbated to her best friend, a girl no less. She was also a little concerned with the mess she had just made, but she was too tired now, so she sank her head into her soft pillow, but she still left a hand on her pussy, gently rubbing it, enjoying the feeling as she drifted off into sleep.