Do some research - in your bedroom

"I think I need to go online to figure this out," you say to Janice as you shake your head and rise from her bed. It felt very soft, but you realize that your butt probably has some extra padding as well.

"Sure, I'll try and search a little myself," Janice beams as she snaps back to attention after gazing at herself in the mirror for a few seconds. You're guessing the awareness of this change has gotten a bit to her as well.

"meet up in thirty minutes to see what we've found out?" you ask her as you step outside. She nods in affirmation and you close her door before quickly moving over to your own room, shutting your own door. You realize the room has changed too, definitely a girl's now, but you still have a computer. Though, just a girly Apple PC... You power it up and while you're waiting for it to start, you get another glance at yourself in a huge mirror.

The Mark part of you is firing you up at the sight of the gorgeous woman. Definitely the sort of girl that men would love to have sucking on their dicks, you think to yourself with a smile. God, what wouldn't you do to have a delicious cock here to nibble and worship.

Oh, it seems you've started masturbating. That's pretty unprofessional, you were supposed to figure out how you became a bitch, not explore what it's like to be a bitch flicking her bean while dreaming about huge dongers ready to fill your mouth with addictive cum. Oh well, you think it's better to get it out of your system than to let it distract you later when you are going to search for other things than pictures of cock...

Oh, the computer is up and running, and your porn folder is giving you a divine slideshow that is sogging up your pussy-ravaging hand. God, you're already edging much harder than you'd ever do as a guy, and you're not even at the facial part of the clip you've instinctually chosen.

Before long, you are crying tears of ecstasy as you moan, squirt and climax. And keep climaxing. For, like, ten, twelve, fifteeeeeen seconds. Give or take a minute.

You blink and realize you're sated... for now. Back to searching why you're a girl. You used to be a guy? Yes, that seems likely. Ridiculous, but somehow you feel it's the truth. Even if a bit inconvenient, you normally would have called some of your boyfriends over to get you your nightcap by now. Ugh, no! You've masturbated, now it's time to work!

You spend a while searching the internet. Well, first you had to clean the computer from some idiot malware, spyware and various things your former/current self never knew or cared enough to get rid of, slowing down your work to a crawl. Search engines seem to have learned of your cock obsession sins, because most NSFW ads are quite relevant to your interests. Ahem, lets hope your Facebook likes are family friendly, lest you have continuous awkward discussions with family friends and relatives in the near future. Fortunately most of the sites containing relevant information for your search doesn't seem to be filled with too many distracting dick pics. To sum things up, magic is real and someone created an app to do pretty much... anything. Welp, that explains the tattoo, you guess.

You also search for "tattoo that changes person when being called something," and a big article pops up on one of the new subforums related to the subject of the digital magic systems, and yeah. It does what it says on the tin. What you do learn is that it's virulent, spreading itself through visual glyphs or physical contact with someone already infected. Only others can change others. Oh, and the more you're changed, the more your original personality fades, if you remember being someone else at all, which seems to be something that can happen if you don't realize it's happening, like you and your sister just did. Jeez, what a pain.

Although, on the bright side, you just figured out what's going on with you and your sister. The bad side, however, is that most likely most of your school and town probably have been infected as well.

You decide to celebrate the event by getting dressed in a cute bikini outfit, fishnet stockings and high heels, checking yourself out in the mirror before moving out towards your sister's bedroom to see what she's figured out.