Then 

Allison looked horrified at the other woman.  “You want to knowingly infect people with a disease!  That's sick.”  Doctor Gyros held up a hand, “Please, understand, the version we are talking about would be non lethal.  What’s more, when you just look at phase one, the health benefits to the infected are incredible.”  Allison crossed her arms.  “Sure, maybe for infected women, I doubt men would consider shrinking a health benefit.”  Stepping toward her, Gyros placed her hand on Allison’s shoulder.  “I mean for everyone.  Look past the shrinking, look at the data I've compiled.”  

After an hour of reviewing the Doctor's data, Allison was convinced that she was telling the truth.  The virus was perfectly harmless with two consenting partners.  More, the resistance to disease and injury in infected men was indeed quite the boon.  They had managed to weaken the mental changes, but not eliminate them altogether.  The physical changes were still caused by sexual intercourse.  What they had changed was the excess fluid generated in the female's breasts neutralized the virus in men.  Regular intercourse and feeding would still be required to stave of the virus.  

Allison saw the potential.  This was the answer.  "Let me guess," she turned, "You need a test couple?"  “We need a test couple.”  Gyros confirmed.  She held up a syringe.  “A dose of our modified virus.  It isn't without risk, but think of the benefits.  No more disease.  Injury virtually impossible."  Allison held the syringe in her hand.  It should be heavier, she thought.  It held within it something that was going to change her and her brother's lives forever.  Possibly humanity too.  Allison jabbed the needle into her arm and with a hiss, injected herself with the modified Blake virus.  

Now

Allison rolled over onto her back taking John with her.  Her impossibly large breasts pancaked slightly as she lay on her back.  Resting her head on a pillow, her black hair splayed into a dark halo.  Her brother fit perfectly on top of her.  He was so light , and yet she could stil feel him.  She enjoyed this as her body tingled from John's boy juice.  It was laced with muted virus that boosted her immune system as well as  filling her with vitality.  Her pussy was still wet and its thick scent filled the air.

She thought about suggesting her brother help relieve her, but decided against it.  It was clear he was struggling with everything.  Allison ran her hand down John's back causing him to shudder and his little pecker to harden.  She giggled.  It was so easy to tease him now.  But she needed to focus, he had made her promise to explain things to him.  “I've taken and infected you with a modified form of the Blake virus.  It limits the mental and physical affects to those that are beneficial to the infected, ie us”  

“How did you get ahold of it?”   Allison’s face sparkled with mischief at the question.  She wasn't quite prepared to tell him everything.  “That’s a different question, little brother.  And tonight you only get one.  That is…unless you’d rather do something else?”  Her hand moved to his torso and began sliding down.  He took hold of her hand, “No!  That is…I want to know about what you’ve done to us.”  She smiled, he was so easy to manipulate sometimes.    “What I’ve done?"  She put a free hand to her chest with mock indignation.  "I’ve given us a chance to be together, brother.  Before, our love would have meant being shunned.  Now?  I can be with you, love you.  Forever!”  She looked at him searching for his understanding.

He was speechless.  Surely this was the disease altering her mind, he thought.  “Ali,” he struggled against his own unease,  “this is a mistake.  Something is wrong, the virus is messing with your mind.” 

Below him, Allison's nose flared.  Of all the thick headed nonsense.  She had explained that the virus' mental affects were limited.  How could he still not see her love for him!

Allison’s hands seized hold of her brother's arms.  She held him without exertion.   “Dammit Johnny,” she berated him, “how can you still not see me?  I love you, you don’t have to be afraid of that love, accept it!.”  He squirmed weakly in her grip.  It was no use, her hands encircled his thin arms tightly.  He wasn’t going anywhere unless she let him go.  “Fat chance of that,” he thought.  Decideing on a last ditch effort, he appealed to her.  “Sis, this is the virus talking, fight it!” 

  Allison’s mouth worked angrily and her eyes blazed.  “I told you,” she rolled them into a spoon, hugging him against her enormous body.  “The virus is modified.  The mental side affects are muted.  I love you, asshole.”  John felt her squeeze him, pressing him into her enormous body.  His swollen member, aroused by this contact, pulsed.  “I guess I’ve got to grow even bigger, to shrink you even more.”  He pleaded his cries muffled into her flesh.  “Oh yes, until your big sister Allison is your entire world.  Maybe then you’ll see my love for you.”