He'll use the back of her under-robe she's wearing

Brocken knew... if he used the panties, he might experience a certain satisfaction unlike what he'd experienced so far... it would, presumably, be something amazing. It might even just work to spend his erection, finally. But... there were too many variables in this experiment his sister had thought up. The answer had never been that he needs an increasingly personal garment from a young lady. The answer had always been how evocative the item was in terms of allowing him to picture his adoptive sister. He drew in a deep breath, then set the soiled panties down next to himself on the windowsill. "You said anything's fine... right?"

"Y-Yeah, as long as it doesn't hurt..." Broomi responded in a quiet voice, feeling uncharacteristically timid. She couldn't turn around... not just because it was too disarming to see her brother looking like a demon, but also, because she thought she might lose her nerve.

"It won't hurt," he reassured her, his heart thudding in his chest as he walked closer and grabbed her shoulders, feeling her jump at his touch.

That brash, quick-witted nun was now completely silent as the tall demon loomed behind her, close enough for her to feel a strange heat emanating off of his member. No, not just that... close enough that she felt it twitching against her back. It wasn't at the right level for a penis' tip... the thing was so over-sized, it was actually patting softly against the small of her back. She tried a few more times to summon her voice, then finally managed. "Brocken...!" It was all she could manage to squeak out.

The incubus moved both hands from her shoulders where he'd steadied her, then placed them at her hips instead. With lustful intent, he ran his hands up and down the curves of her hips, then began to slowly rub his gorged incubus cock between her buttocks, against the soft fabric of her under-robe.

"B-Brocken!" she gasped again, reaching out her hands to grab hold of the bed-post in front of her, the other clutching onto his bed-sheets at a lower elevation. From this position, her backside was out towards him... She didn't have much in terms of curves, but her butt came the closest to providing some semblance of womanly shape.

Instead of feeling the erection going away, Brocken felt it throbbing madly, harder than he'd ever felt it before. Part of that was because he wasn't letting it release... selfishly, he held it in, feeling that it was his only ticket to enjoying this once-in-a-lifetime situation. "Sis! I'm just... I'm just using your robe...!" he reassured her, now grinding himself harder against it. The heat she was feeling now wasn't just that of a boy's penis against her ass; it was a hot, curved incubus cock, the kind that few faithful nuns had likely ever felt.

"Dummy! I-I'd let you use it if you'll just give me a sec to take it off!" she protested, though she didn't try to break free from his embrace.

"No, it's... it's better this way!" Brocken grunted in reply, his fingers now moving from the sides of her hips to fully clutch her ass beneath the robe. He felt her gasp and it excited him... he thrust his cock harder, encouraged by that response. As he moved with greater and greater enthusiasm, he began to note the tearing of the under-robe... just as he'd destroyed the habit with his earlier masturbation, he was ruining this garment now. "Sorry, Broomi! I think we're going to have to replace this one too!"

"We will!" Broomi reassured him, knowing she ought to be angry... but instead, she could tell from the wetness of her pussy that this taboo circumstance had her every bit as excited as her brother was. "Don't worry about it!"

"Th-The robe is working really well! I think I should use the robe from now on!" he groaned, using that pretense to gather up handfuls of it, which actually meant pressing his palms and fingers into the softness of her ass. By this point, the knee-length robe was bunched scandalously up to just below her rear-end, showing off her skinny, lightly freckled legs as the bed creaked under his movements and the knees bent against it. He moved one hand to touch her thigh beneath the hem of the garment... then felt moisture. He gulped.

"That isn't part of the robe..." Broomi corrected him, grinning teasingly over her shoulder. Her freckled face was red from cheek to cheek and sweat rolled from her bushy brown hair down her slender neck. He could see, quite clearly, that she was horny for him.

But that smile! His adoptive sister's smile teasing him while he rammed his cock up and down between her ass-cheeks! It was too much. His incubus dick finally gave out, beginning to release a massive load over the back of her under-robe. The propulsion was so great that the semen ended up coating not just the butt of her robe, but also her back, her hair, the cheek she'd turned, and even the wall behind her. "Aaaah, fuck!" he cried, feeling a release like he'd never thought before... The initial stream had been so powerful, but it just kept coming. His sister, his bed, and even the back wall of his room were thoroughly coated with the stuff. "It's... It's spent! I'm actually spent!" he groaned, stepping backwards and falling flat, where Broomi had fainted earlier. He'd never actually been spent before. It was another of many new experiences for him today.

Broomi stood shakily to her feet; she'd been drenched and the under-robe she wore was torn around the small of the back and the buttocks, causing the crack to show through indignantly. When she turned... her face didn't match Brocken's at all. He was wearing a face of utter weariness... she was wearing an aroused expression and breathing heavily, drinking in the details of the incubus' handsome body, so handsome she could forgive the weird dick. "I think... We found what works," she gasped with a smile, teasingly poking his shaft with her toe. Unfortunately for her, it didn't spring back into action.

"No, Sis, I mean it... It's really spent," the boy moaned, clutching his hands over his face. "Thank you... Thank you so much... I've never..."

"What do you mean?! Like, it's not coming back?" Broomi questioned, already cursing her bad luck for letting that end at a butt-job.

"No, I uh... I think it'll come back eventually. It's not that different from a regular dick, just cursed to be way hornier and full of too much jizz," he grumbled, now sitting up to see that she wasn't just teasing, she was still terribly horny. "Sis... Sis, just leave it for now... And later we'll uh... We'll think of what to do. You should go take a shower... and I'll... clean all this up... somehow."

Broomi pouted, pressing both fists on her hips. This didn't seem fair... She could worry about the implications of doing this sort of thing with her adopted brother later, but for now, she was so horny that she herself felt like a demon. Maybe... if it really was like a human's dick, perhaps it would jump back into action with the right offering...?

Or maybe she should just go take a bath and cool down. There was a part of her that knew she was way too horny for her brother right now and things were going to be even more complicated than if he failed to achieve knighthood, if she continued pursuing the path she was on now.