Public Nudity/Act oblivious like you're still in your towel
So far you haven't given away that you're aware of your nudity, so maybe you can work with that.
"Um Kevin? You still there? What's with the surprised look? Is it because I'm still in my towel?"
That seemed to shake him from his stunned state. He looked up at your face, cheeks tinted red and brows furrowed in confusion, and stuttered out a reply.
"Your towel? But, your towel, it, well, uh-"
"Oh! It's probably a little dirty from the batter."
"Batter?"
"Yeah, pancake batter. I went and made you some just before you called me, that's why I'm still in my towel."
You cock your bare hip out and put a hand on it as you continued.
"Well don't just stand there, they aren't getting any warmer you know."
The complete change in topic compounded with your (seemingly) ignorance that you're on full display for him seems to leave Kevin at a loss for words, barely able to mutter a reply.
"Um, okay, I'll just finish up here and go get dressed first..."
With a beaming smile you clap your hands together and give a little hop in place, keenly aware of the slight bounce in your boobs, and how Kevin's eyes seem to follow them.
"Great! I'll go finish setting everything up!"
With that you turn to leave, being extra careful to step over your fallen towel in a manner that doesn't make it look like you're intentionally avoiding it. As you turn you stealthily glace down at his waist, noting the sizable bulge beneath his towel. He either has amazing self control or is too conflicted by seeing you naked as it's noticeable, but nowhere near fully erect. As you turn fully you find yourself noting that his member must be quite large if that bulge was any indication.
"A-Aren't you going to get dressed?"
You pause in the doorway and look back at him, being careful not to let your gaze fall too low to where he might think you saw the discarded towel on the floor. You give him a playful smirk.
"You must really be used to living with a bunch of guys if seeing your little sister in a towel bothers you this much."
With a giggle you leave the bathroom, closing the door behind you.
Once in the clear you let out a sigh before struggling to contain your glee.
OhmyGodOhmyGodOhmyGod! Kevin saw me naked! Completely naked! And the best part is he bought the act and doesn't think I know what happened! Okay Claire, calm down girl, go take care of the food first. Celebration can come later.
Shaking your head lightly, red locks swinging back and forth, you descend the stairs wearing nothing but the smile on your face.
What next?