As the 38 year old "twin sister" of Yang wandered to the library in search of said "twin sister", she was oblivious to Yangs presence merely twenty feet behind her. She was following Rang from a safe distance, ducking in and out of cover to remain unseen by her twin.
Yang couldn't believe her luck. She had bought that cheep good luck charm on a whim, she hadn't believed the travelling salesman who she had bought it from when he told her that it was enchanted to make its masters desires a reality and all she had to do was wear it long enough to be recognised as its master. But now? Well, now she could see the results in the form of a look-alike that stood at five foot eleven, with tits as big as watermelons and ass cheeks to match, all of which were crammed into a duplicate set of her current outfit that was way too small on the supermodel in front of her.
This had all started for her when she saw Jaune pick up her necklace from a distance. She was about to call out to him, since he looked like he was going to set off to return it to her, when she noticed that he was changing. When the changes stopped, Jaune had become an exact duplicate of herself and had set out to find her "twin sister", Yang, while Jaune himself/herself went by "Rang".
All Yang could recall at the time was that she really wanted the necklace to be back in her possession, so it seems that the enchantment took that literally, although in a more roundabout way, and made the person in possession of it into Yang. However despite the desire being fulfilled, it could not make Jaune into Yang, as it could only have one master, hence Jaune became the closest thing to Yang, being Rang. Then there was the other change, which was just a thought that usually goes through Yangs head whenever she sees Professor Goodwitch, 'I wonder if I'll be that stacked a her age'. And so Yangs desire, born from her curiosity, witnessed her double grow two inches in height, go up three whole cup sizes and nearly triple the volume of her ass cheeks.
Yang was shaken from her recollection as she heard Rang open and close the door to the library. Deciding that since everyone saw happened to Rang during Goodwitch's class as normal, she might be immune to any potential consequences of seeing how far she could take this and so, followed her twin into the library to cause a little trouble of the steamy variety. As she closed the door to the library behind her as quickly and quietly as possible she scoped out the area.
She quickly saw Rang trying to get through the gaps between the book shelves without knocking anything over, only to fail miserably and scramble to clean the mess she made. Yang looked elsewhere now that her "twin" was accounted for, to see who would volunteer for some experiments. She somehow knew that she could alter what Rang wanted to do but she still didn't know how far her control over the body alterations went, so she figured that some testing is in order.
She saw Neo (though she was disguised by her semblance and Yang knew her as Nicole) just lounging around like she owned the place. And she saw her partner, Blake, reading one of her many books all by herself.
Neo(Nicole) and Yang never really talked, and not just because she mute obviously, but they had history over the last month or so. Neo constantly pranked her, it was just small things sure, but the fact that she would just stand there and mime that she was laughing before vanishing with her semblance before Yang could even get a word out was infuriating. Maybe she could get Rang to sit on her and enlarge her twins dump truck booty even further. The only issue with that is, well, for all Yang knew she might be into that.
She see if she could get Rang to start lactating, that would be a sight and it might kill two birds with one stone and enlarge her breasts further. On top of that, Blake was known to be a bit of a milk fanatic. Whether or not it's due to her being a cat Faunus was irrelevant. Regardless, Yang knew that out of all the people that would say no to being breastfed by a blond goddess like Rang, Blake was not one of them.... probably.
Of course these were only the first two people she's seen in the library so far, so there's bound to be other people to experiment with if she just kept looking.
Now then, time to make a decision.