Why not the attic?:
"Lynn what are you doing?"
The jock yelped and fumbled backwards, her little box fort of privacy in the dark corner becoming a box pile of shame.
"Nothing!"
"It doesn't look like nothing." Lucy's monotone gained an edge. "It looks like you were trying to take my corner for your own."
"Look Luce-"
"What was that buzzing sound you were making?" The smaller girl seemed to glare through her bangs.
Oh shit…
An 'early education' of one of the younger sisters carried a month's minimum grounding.
Thinking quickly Lynn hid her 'friend' behind her back and thumbed the off switch. "I was just… brushing my teeth! Y'know what trying to get in the bathroom is like!"
"In the attic?"
"Yep!"
"With no water?"
"Psshaw, I'm Lynn Loud; I don't need water to have clean teeth!"
Lucy seemed stumped for a moment. "Lynn; what are you hiding behind your back?"
Lynn quickly plopped her oblong boyfriend into her waistband and showed her empty hands. "Nothing!"
"… and why are you wearing a skirt?"
"Because it's hot today; obviously!" Lynn grinned, because I can just flip it down if I get caught; who says I'm dumb?
Lucy seemed to look through her. ". Sigh. Obviously you aren't going to tell me the truth. But that's nothing new. Just go someplace else while I fix the mess you made."
"Okay Luce, sorry to mess up your stuff!" Lynn waved goodbye awkwardly and made to turn to the stairs-
And found herself butt-first on the floor again; having stepped into her Mother as she came up to investigate the crashing from upstairs.
"What on earth are you doing up here Lynn-"
Rita's attention was suddenly taken by a buzzing sounds being emitted from a cylindrical object that had fallen from her daughter's pants and landed on it's highest setting.
"She said she was brushing her teeth." Lucy's voice condemned.
There was a moment of almost silence as Lynn felt her summer dissapear.
"Oh, well that makes sense." Rita smiled, seemingly placid as she looked to her fifth youngest. "Lynn seems to have dropped her toothbrush after all."
"That's a strange toothbrush."
"It really is, and she shouldn't have been using it up here where she could make a mess. Why you go downstairs while Jr and I have a little chat." The same warm smile again.
"That's okay, she didn't a make mes-"
"Have some lunch dear" Rita's left eye twitched, and Lucy vanished. "As for you Jr-"
"I'm sorry." Lynn Loud knows when it's time to fold.
"Oh sweetie." Rita calmly stepped forward and carefully switched off Lynn's 'friend'. "Not as sorry as I am.
But you'll get there."
The vents?:
Lori slammed the vent grill closed in front of the Jock's face. "I swear Lynn, if you even think about it I will-"
"Get in line behind me," interrupted an unusually irate Luna. " 'Cause I'll make a drum out of your football skins!"
"What's the big deal guys?" Lynn snapped. "I'm ju-"
Lori's hand snapped forward to pull Lynn upwards, then with a nod to Luna the two of them grasped her by the legs and shook the smaller girl like a bag of potatoes. Instantly assorted sports memorabilia started falling from her pockets, along with a flesh colored egg like object.
"Just going to make a scene." Lori dropped her, forcing Luna to release Lynn upon her beanlike head, The eldest delicately grasped the object with as few fingers as possible, before throwing it out of a window in disgust.
"HEY! I just got that after-"
"After Mom caught you last time, we know dude," Luna sighed, and took a breath to calm down. "But goin' in the vents? What kind of idea is that?"
"A great one!" Contended the fallen athlete. "I can still fit, and Lucy agreed to leave the vent abo-"
"No. Stop there," Lori halted her with a palm. "I don't want to know. The bottom line is that your 'plan' would have literally failed in the worst way possible."
"Yeah dude, didn't you think this through at all!" Luna exclaimed with both hands.
"Think through what?" The jock unfurled herself to stand to her full (utterly unimpressive) height. "No one else is in there so I'm not gonna get interrupted or anything!"
"Think through literally anything about how this was supposed to work; obviously," Lori facepalmed. "Look, what do you think is going to happen right after you 'get going'?"
Lynn scratched the back of her head. "I dunno; I'm gonna-"
"You're going to get loud dude; seriously I don't think anyone gets into it like you!"
"I can be quiet!"
"You literally can't," scoffed Lori, sadly remembering the times she had caught Lynn 'being quiet'. "But even if Mom and Dad think we're just haunted by the worst ghost ever, they're not going to think that when the smell gets to them."
Lynn flushed. "I d-don't sme-"
"Dude, it's part of the deal; we're not shaming you for it."
"Yes Luna," Lori advanced to look Lynn in the eye. "Look, everyone who 'enjoys themselves' gets… 'fresh', that includes you. It includes Luna and it literally includes Mom and Dad-"
Luna interrupted "You too Lori. But I'm not sure if bo-"
"The point is; they've had eleven children. Do you think either of them won't recognise that smell literally the second it reaches them?"
"…. I thought Lincoln might be adopted?"
"Arghh!" Lori felt her 'big sister' headache coming back "Possibly eleven children, and when the smell comes through, and it will in every room in the house because that is what the vents are literally for, they're going to do a headcount to find who made the stink. Even if you're really fast, Mom will be able to tell because it'll still be strongest on yo-"
"I brought deodorant!" Lynn pointed at the pile of Lynn-nacks.
"Then she'll know it's you because no one else smells like it and you obviously hid it." Lori sighed. "Trust me, this is not the first time I've had this conversation."
"Who els-"
"Literally not the point Lynn. Just tell me you're not going to try this again." She crouched to look into Lynn's eyes, trying to make her understand.
"How can I?" The younger teen snapped. "You threw out my-"
"Lynn, please dude." Luna pleaded over Lori's shoulder.
"… Fine." The jock relented. "I guess I don't really have a choice."
"None of us do," Lori stood up in relief. "Now let's get to bed befo-"
"RITA!" Lynn Sr's voice echoed from outside, the second floor window clearly showing his disturbed expression below as he stood in the garden bearing Lynn Jr's toy in two fingers. "WE HAVE A CODE PINK!"
The younger two glanced at Lori, who sweated bullets. "I-I-"
"You fucked us all Bruv."
What about tying their tubes? Maybe that will calm them down?:
"So you see; it would be entirely pointless," Lisa finished her demonstration and switched off the projected diagram in her room, her audience in despair.
The previous lecture outlined how five vasectomies and two tubal ligations had failed in their fecund parents, only to be utterly overshadowed by Rita's hysterectomy reversing itself.
"That's literally not how that works!" Declared Lori. "Organs don't just-"
"Negatory elder shister." corrected Lisa. "In most cases you would be right, however our family has a mutant gene inherited from our sprightly grandfather that allows for faster and more effective repair and recuperation than most other homo sapiens. You can thank it for being the sole reason our weekly 'disagreements' haven't put us all in wheelchairs."
The genius turned to glare at Lori, "that said; outright neutering via ovarian removal is still a possibility I would be happy to confirm or deny."
"Got it."
"So... mom's baby factory still works?" Lynn queried, rightfully unsettled at the idea of ever more siblings.
"Indeed fifth eldest sibling unit, and sadly for us the same condition will likely fall upon each of us as well." Lisa adjusted her glasses before putting her projector under her desk.
"I dunno dudes," Luna countered. "Pop Pop can rock out in his eighties; maybe havin' a workin' kid factory no matter what ain't so bad-"
"Shut it Luna," Lori almost screamed. "You're gay- you don't risk nine months or a coathanger every time you 'cuddle' with Sam!"
"Bi actually, but yeah I get your point. Oh well, guess I'm just lucky 'cause Sam LOVES the Loud lust and I don't have any down side-"
Lisa interjected with a raised finger. "Actually my sibling units I must inform you that from my studies it would appear that as a consequence of our superior healing abilities it would seem that menopause is... greatly delayed if not absent entirely in our female relatives. Indeed if not for our grandfather's dentures I would assume full bodily restoration would be possible well into our ninth decade, our uteri being no exception."
"... What?" Luan's mind refused the fact.
"We're sailin' the Red Sea forever my dudes."
"DANG IT!"