Prove it.

"The first week we were here I took Chad to the walk-in clinic if you really are Chad, why did I take you?" He made you promise not to ever mention it again, and this is a great opportunity to remind him that you remember it.

"Hey, that isn't cool, man! You promised never to bring that up again!" She's not very happy that I'm bringing it up again. I'd better not laugh out loud.

"Well? I know Chad wouldn't ever tell this to anyone else, so I want you to say it out loud" she is either going to be pissed and not tell me or embarrassed and reluctantly tell you.

Her face goes bright red and she looks away from you "I needed to go because I looked up my symptoms online and they said I probably had some kind of cancer or worse."

"And what was it, really?" You're really holding back laughing now.

"I... I was constipated" her voice trails off and her entire face turned red and she refused to look at you even when you tried getting into her line of sight.

"Okay, so you are Chad, but how?" You start to pace a little as though you're in thought, "you should take off your clothes, we need to see just how much you have actually changed and see if everything has changed."

"What are you serious? You fucking pervert!" I guess bringing up the clinic wasn't a good idea, especially if I wanted to get her naked.

"Listen, if you want me to help you I need to know how much you have changed, I mean I might not be able to figure out how to help you but I can help you figure out how server your erm gender switch is" you say trying to convince her "Oh and what should we call you? I feel weird calling you Chad when you're a girl"

"I guess you could call me Abigail, it's what I was going to be called if I was a girl when I was born, and I'm still getting naked in front of you, can't we like, go to the hospital or something?" 'Abigail' is obviously not keen on the idea of me checking her body out.

"NO! You can't go to the hospital, they wouldn't believe you and think you're stupid, or if on the off chance they did believe you they would lock you up in some kind of military base and do weird experiments on you, have you never seen movies about mutated people? They poke and prod you until you are dead trying to figure out how you changed" you are actually a little concerned at this point because although no one will believe her she is likely to end up getting into some kind of trouble if she goes and starts telling people that she was a guy that was turned into a woman.

"Oh, you might be right, I don't want to end up like E.T." She has a worried look on her face, she wasn't the brightest as a guy I doubt she will be much smarter as a girl: "S-So is there nothing I can do? Other than stripping off for you to ogle me!"

"You know I doubt you look that much different from Emily naked so it's not like it would be anything I've never seen before," you say nonchalantly.

"Wait, what? Why have you seen Emily naked?" today doesn't seem like Abigail's day

"Relax we haven't had sex, we just fooled around a little" Although it isn't a complete lie It is definitely not the entire truth, in truth you've only seen her tits she was never very shy about getting those out for you, in fact, she's gotten them out just to surprise you and make you blush. As for fooling around, that wasn't entirely a lie, one night you ended up fingering her while she jerked you off, but neither of you took any of your clothes off you just put your hands in each other's pants.

"Are you serious? She is my sister!" Still pretty pissed

"The only reason we know each other is because me and Emily are such good friends and what we do with each other is our business, but that besides the point, there are two options you let me look and see how far your body has changed or I could give Emily a call and ask her if she would come over, either way, you're going to get your body checked out" you put on a more demanding tone to reinforce that you know what you're doing.

"No, but why? I don't want you looking at me, and if Emily comes here she will take make fun of me, wait, give me a minute, maybe I can think of something else" she is just stalling and trying to weasel out of it, you were sure mentioning that bringing Emily in would get him to agree to strip for you.