From big brother to baby sister

The beginning

One day Dustin was lying in bed when his little sister,  Bethany entered his room. Now the age gap between Dustin and Bethany was pretty significant. Dustin was 17 and Bethany was 12, and Dustin wasn't exactly the kindest to his younger sibling. Little did he know it would all come back to him on that very same day. "Do you want to go play hide and seek?" Asked Bethany. "Why should I, why don't you go play with your dumb baby dolls or something" I laughed. My sister exited the room with her head hung low. Later that night Dustin was grabbing a snack from the pantry when he heard some clutter downstairs, instantly realizing it was his father. Dustin's dad was an inventor, he had created some of the strangest, and most bizarre machines the world had ever seen. Trying to ignore it, Dustin walked back up the stairs to his bedroom to go to bed. The next morning Dustin's parents announced that they were going on a business trip out of country for a few weeks. Leaving Dustin and Bethany alone. Bethany pleaded with them not to leave, not wanting to be left alone with her older brother. Nevertheless they took off after lunch saying their goodbyes and telling them to be good. Later that night while Dustin was sitting on the couch watching tv, Bethany snuck up from behind him and yelled "TURN AROUND."
Dustin did as she asked and noticed she had a small ray gun looking thing pointed at him. She fired and Dustin instantly began to shrink, his clothes getting baggier and baggier until they literally fell off of his body. When he stopped shrinking he looked up at his sister, who looked way bigger. When he tried to speak he realized his voice had become way more high pitched and squeaky "WHAT HAVE YOU DONE TO ME" he screamed. "I turned you into my little baby sister" she said smugly, "I can't have you as my annoying big brother for the next few weeks." She then picked him up effortlessly and pulled a syringe out of her pocket, and injected it into Dustin. "Night, night baby" was the last thing he heard before he blacked out.