Alrighty! Another potential Gender Bender romp. Lessee, you want honesty, and thus honesty ye shall have.
First off, it's wonderful that you're giving creating a game a go, to begin. It's not easy by any stretch. You seem keen on retaining a bit of light humor and fun to the overall vibe, and that's wonderful.
And now to nitpick things in no particular order.
First off, if the Asian gal is supposed to represent our protagonist that's been gender bent, that seems way shorter than 5'6". And not that it's a huge matter, but the sister is clearly a blonde, so if that's indeed our pov, then it means for a mixed household or adoption.
... Which isn't the worst thing, as there are legion beautiful Asian adult actresses and such that could be the mom, for example, which would be another way for your game to stand out from the pack.
If you go 'submissive' dealing with the sister, you have this longish scene of coming clean just for her to use you, and then tell you to get your boyfriend to buy you nicer clothes. What. Is this mixing the scenario as a 'dom' where you are pretending to be the girlfriend, or something?
The interactions with the best friend seem like a fun gateway into potential scenes, which makes absorbing the gem almost unfortunate if that's the end of Rory's misadventures.
One style or element for this sort of setting (A gender bender game), while tricky to write and is thus sadly, terribly, horribly absent most of the while is:
The best friend angle where *both* characters end up changed, and dealing with the world as they mess around. If you are establishing the logic that the protagonist can shift between male, female, or in even in between (the first change) along with the caveat that the world is now behaving with what is essentially porn logic, and the protagonist and / or friends are at the center of it, then my not so humble suggestion is roll with it.
Maybe give yourself some wiggle room or plot armor that when things get frisky there is a potential out or a way to mess with others memories to a point. Especially if the appearance of the MC is 'fixed' as they are exploring their sexuality.
But for example. Let's say the protagonist can shift, and using the power of the gem that has merged with them, they can change up their best friendo too, because that's way more fun. Maybe just for a little while at a time, but it grows longer and more stable the more you do it (and they enjoy it). They lamented that the change was based off of lust, but clearly the answer is to get them sexed up. Or maybe it's easier to change them because they also were 'attuned' to it early on with the experiments.
Anyway. Why not play with a setting where both characters, discovering they are now a pair of knock out beauties, are still very much horny lizard brained young adults and start doing everything they 'think' a duo of sexy gals should do, often with hilarious or unfortunate results? (Because again, porn logic.) Dressing up in slutty outfits "paying" for them or suggesting a discount, going to clubs and parties without waiting in line (only to end up needing to flash their tits or suck the doorman off), pizza gets delivered only for it to be by a AAA hottie or hunk, yada yada. Have them attend school, but events constantly conspire to get one or both of them banged by the school and faculty.
Maybe make some sort of impressive cash goal, where the protagonist, friend, or even the sister (or all of the above) decide to break out into streaming or whatever to raise the money necessary. Really, the sky is the limit.
Last little bit, as I see you have a dom sub split. Why not have an option where you come clean to the sister, but you don't act like a delicate little flower? Like, other you (the female one) seemed pretty happy and confident. The (somewhat) tired trope of 'oh no, don't tell anybody, I'll do anything you want' is fine, but why have a alternative of simply saying something along the lines of "... There's no way in hell you'd do that, I know for a fact you'd never hurt me. If you wanna bang, just say so, 'sis'."
So, sister starts getting lewd, and the PC just rolls with it, as stupid a plan as that might seem. Hormones are stronk. You meet the siblings charge head on, and before they know it, both 'sisters' are lost in a tangle of fingers, tongues and whatnot. Sister mutters you may have just ruined her on other girls.
Another story beat that seems to be rarely done is the protagonist just *enjoying themselves* in this sort of game. Why not have the sister offer to show her the ropes on how to girl (tm), often with lewd or surprising results?
But in the end of the day, these are just suggestions. Remember to have fun, and if you can, explore whatever gets your motor running, yeah? That tends to be where the best inspiration is laying dormant. Best of luck!
