Jess had succeeded to stay out of sight from the rest of her family for months now. She had grown a little taller but was still flat chested and afraid of how much her little sister may have grown. One day she heard her mom, dad and sister leave the house and Jess decided to go snoop in Chelsea's room. She found a bikini top and bra's that were D cups. Her mom was a D cup but she was pretty sure these were Chelsea's. She found a picture of Chelsea her mom and the some of the girls from the high school soccer team standing on the sidelines of the high school soccer field. Chelsea was astoundingly standing nearly her mother's height and was almost the tallest of the other girls in the picture. All the girls had their sleeves rolled up and were flexing their biceps in a silly pose. Chelsea's arm was twice as big as all the other girls and her legs were super thick pure muscle. Jess suddenly heard her dad and Chelsea talking and walking in. Jess scrambled to leave Chelsea's room but it was too late. Chelsea spotted her. Jess looked at her in fear, she was a monstrous size. She was only a couple inches shorter than her father and her body was all muscle. She was wearing skin tight super short spandex shorts, an under armor female spandex-like workout top that only went around her huge chest. Chelsea quickly ran over to Jess, Jess's head just barely higher than Chelsea's belly button. Chelsea put her hands under Jess's armpits and lifted her up like a child and held her up to the ceiling. "Look it is baby sister Jess" said Chelsea. Mom walked in "Oh my, Jess you are still not growing, this is ridiculous, look how amazing Chelsea is" she said as she walked over to Chelsea standing about 4 inches shorter. "I am as strong as dad now, he won't admit it but he was fully ready when I beat him at arm wrestling this morning" said Chelsea. "I wasn't ready, I swear" said their dad. "Rematch then daddio" said Chelsea. They set it up. "I will let you get a head start dad." said Chelsea. "Go" yelled mom. Chelsea's dad exploded his muscular arm and moved Chelsea's back a little bit. Then Chelsea's arm visibly burst with muscle and she steadily pushed him back and won. "Boom! It's ok dad, I am kinda huge." said Chelsea as she stood up and flexed. Their dad was very fit and muscular, but it clearly appeared Chelsea was now larger in size muscularly. Over the summer Jess finally got her period and two tiny tic-tac sized breasts. She now stood 4'10" inches tall going into freshman year of high school. Jess overheard her parents talking about how it was great when Chelsea was just a big athletic girl for a kid, but now they could not control her because she was way overpowering. Jess looked out back as she could hear her parents and Chelsea talking. She should not be shocked anymore but was as she saw her father only up to Chelsea's shoulder in height and clearly way smaller. Chelsea's arms were as thick as his legs. Dad was talking sternly to her and she smiled and picked him up and held him over her head with two arms. He was squirming but she just laughed and said "Oh dad, you are so small and cute, just shut up, I will do what I want." she then walked over to the pool and threw him in. He jumped up and said "Chelsea you do not treat your father like that." Chelsea grabbed his head and held it under water and was smiling. Her mom started to pull at Chelsea from behind but it looked silly. It looked like a person trying to push over an oak tree. She let him up and he gasped and she pushed him back down. This time so long it was a little scary. He let him up and he coughed and gagged and gasped desperately for air. She grabbed him by one arm and lifted him out using only one of her arms. She folded his arms over his chest and held him against the garage with one. He could not even move. "Now, I could kill you with one flick of my wrist, and I still have this arm free to do curls." she said. "So I will treat you however I want"