Then 

Allison had watched her brother excuse himself to take a leak.  She watched enraptured as the various displays flashed before her.  What had happened seemed insane.  The Blake virus apparently interacted with the host body in such a way that it mutated it.  What’s more, the mutation appeared to be dependent on sexual coupling.  In fact, most of the changes seemed dependent on it.   For female hosts, increased size, enhancement of sexual characteristics, and hyper libido.  For male hosts, increasing penile size, a hyper libido, and the production of low level attractive agents.   Looking at the changes, Stage one wasn’t all that bad.  Male carriers shrank, but they became healthier as they did so.   Female carriers on the other hand grew larger as well as healthier.  

Allison realized that any benefit conveyed by the virus to women came at quite cost.  Mentally, the Blake virus affected both genders.  For men, it created a sense of ease and wonder.  A high susceptibility to suggestion.  Most of these changes were muted until the final stage.  For women, it was much worse.  Carriers developed intense feelings of attachment, arousal, power, greed, dominance, and more.  The graphs charting the various mental symptoms were the most troubling.  The mental influence set in early at stage one and continued to worsen evenly with each progression.  

Another thing that Allison hadn't realized was that without a partner, a sole carrier of the Blake virus would increasingly worsen.  The disease needed the male female relationship.  Without both, it wouldn't survive.  The disease died off in the surviving carrier with a mortality rate of 50%!   “That explains the sex affects,” Allison thought.  The physical improvements helped secure a partner.  Quarantine would be the key stopping an outbreak. 

Infection occured when individual’s bodies came in contact with a carrier's virus laden fluids.  The virus also created byproduct of itself due to the changes it made.  This byproduct was released by the carrier, men in their semen and women in their breast milk.  The introduction of these byproducts of the virus in the opposite sex’s body led to speading up the disease.  By the final stage, it was lethal.   The disease overloaded the systems of the infected and destroyed their minds.   

She was glad her older brother had brought her.  He would be leaving soon and Allison didn’t want to see him go.  John was her brother, she was adopted, but still she knew that her feelings for him weren’t 'socially appropriate'.  Allison couldn’t be with him.  It would look weird.  She had run at of excuses to be near him.  

“I see you like my exhibit.”  Allison spun around and saw a short woman in a dress smiling at her.  “Yes,” Allison shook the woman’s hand.  “It was very informative.”  The woman titled her head at Allison.  “If you don’t mind my saying, you don’t look entirely happy…”  Allison sniffed.  She hadn’t meant to show her emotions.  “It…has to do with something else.”  Allison’s new friend reached over and put her arms around her shoulders, “Tell me all about it, dear.”  

Now

“And so I did.”  Allison reclined on her bed with her feet, heels and all, dangling over the mattress.  She had outgrown her bed some time back.  Her dark hair splayed behind her on the pillow.  Her big brother John was sitting on top of her bowlegged.  He struggled to straddle his sister's massive torso.  John's outsized erection lay on her soft tummy as he leaned forward into one of his sister's fat titties.  His cheeks puffed out as he suckled at her thick rubbery nipple.  The milk had come fast.  It had been suprisingly warm and sweet.  He'd been unable to resist and had greedily begun sucking down her nectar.  

Above him, Allison was clearly enjoying her brother's attention to her juicy tit.  It felt amazing to release her milk, to feel him suck it down.  And his body.  On her, so close and so small.   He was  adorable.  His right and left hands pawed at her bare breast, trying to pull it towards him.  Its size was such that he was having to exert himself.  At the same time, his penis was hard and he was grinding it against her stomach.  He was fully consumed with passion for her and yet it was nothing like when she had experimented with boys before.  They had been big and rough.  Powerful and heavy.  John was practically dainty and gentle.  She knew part of that was his shrunken form, but still.   Allison felt so sexy.  She was in control, the one who was heavy and powerful. 

She basked in all these feelings and let her arm leave John's back.  He was so lost suckling on her that he didn't notice.  She trailed down her pubis to her warm, wet pussy.  She gasped in pleasure as she played with herself.  This was her brother, the man who used to pick her up and swing her around.  Who carried her when she was sick.  Who when he didn’t want to pay attention to her just batted her away.  She'd tried again and again to get him see her.  And now....

“You can’t ignore me now, can you shorty.”  She breathed out as a small orgasm shook her.  John felt the warm giantess shudder under him and paused .  His little mouth was open and milk was running down his chin..  The poor thing, he must be so full.  “What?” he asked her.  Allison lifted the hand that she had been using to stimulate herself back to John’s cute little butt and gave it a light smack.  “Tut tut, did I tell you to stop?”  She gave him a look that indicated she didn’t want any response other than for him to return to sucking.  He gave her a final pleading look before obeying.  She gasped.  He really was good with his mouth.  So good she had just cum from breast feeding him.  Her thighs were slick and her sheet no doubt drenched. 

“A-anyway,” she struggled to break free of her lust.  He had done what she'd ask.  Good behavior should be rewarded.  “I spoke with someone a while back at that exhibit you took me to.  Her group had developed a modified version of the Blake virus.  One that isolated the carriers to stage one.  Additionally, the virus fluid exchange is different.  Drinking my milk keeps my little boy from shrinking when he goes…” She paused.  John couldn't restrain himself, "what?"  She licked her full lips.  “Pop.” she made an over the top popping noise with her mouth.   John's cock twitched and it took all his effort not to spray his sister's stomach with his cum.  Still, his hips twitched and he was red faced.  Allison’s eyes sparkled, it was so hot he was rutting on top of her.  But he must be quite frustrated, full of cum all for her of course.  “Sadly,” she said as she rolled over, “Your cum doesn’t make big sis Allison grow if I feed you my milk, at least for 24 hours.”  She pouted her lip and lowered her chest.  John had fallen off of her and was now pinned.  Her enormous tits pushed out and together by her elbows fell heavily onto him, pining him to the bed.  John groaned.  P-please…” he begged her, desperate for release.  Allison moved down, her gigantic chest pushing into him, burying him in her softness.

“What,” she asked teasingly.  She knew very well what he wanted, but he was going to have to earn it.    “W-e-llll,” she kissed John’s stomach below his belly button.  So close.  “I guess I could help you.  But f-I-rst…”  John breathed heavily.  “Anything…”  Allison smiled.  “I need you, Johnny, to ask me."  He was desperate, he would have said anything.  "I want you to  say, ‘I want you to blow me Sis.;”  

It was a miracle he didn't blow it right then and there.  John’s eyes had rolled back.  “I n-need you to blow me, big Sis.”  Allison’s eyes shot wide.  He had added in big.  Hearing him acknowledge it, that made her very horny.  He would have to help her with that.  “Oh I will,” her eyes narrowed.  “I’m going to rock you, little brother.”  

Allison started kissing down John’s pubis, to his shaft.  She stopped, breathing onto John’s dick.  She giggled as a bit of prep-cum shot up and hit her in the face.  She’d always liked to play with the hose as a girl.  Opening wide she took John inside her mouth.  She couldn’t believe how even at his tiny size, she couldn’t suck his whole length.  She wondered whether she could fit all of him inside of her.  Definitely something to try out later.  Beneath her, John was writhing with pleasure.  His hands reached out and ran thru her hair.  Finally, he began to buck his hips as he came inside her mouth.    Allison took every bit.  Every drop.  She looked him in the eyes as she did it.  Then she raised her head and looked down on him.  John’s mouth opened as he watched his sister smile and then *gulp* swallow his cum load.  The bit of pre-cum still clung to her face. 

John wheezed but noticed that he was not shrinking.  “I’m not…shrinking?”  he said with a look of dumb wonder.  Allison giggled.  “Of course not,” she licked her tongue out and lapped up the remaining jizz on her face.  “I promised you if you were good you wouldn’t.”  That meant he didn’t have to shrink any further!  He looked up at her wondering what came next.