This sister is goth/punk and going to have some super goth friends and some punk friends, and basically be open about stuff, be very hard to get, very demanding and cynical at times.
A subtle detail that I may not have expressed in sufficient clarity is that those are two very different personalities, and you should pick one of either. Even though teenage girls themselves would disagree, punk and goth are just phases, while the personality changes more slowly. For example, the punky chick is essentially a hippie and was sexualized at an early age and is sexually experienced, has "friends with benefits" and generally not care about fucking her brother as much as caring about fucking a guy she shares a house with and may interfere with her ability to see other guys. Meanwhile the gothy chick may well be a virgin who overcompensates this with the oversexed look, erroneously assuming elaborate clothes that are hard to get in and out of would help her, and her sexual experience is mostly based on a touchless thing, such as watching semi-gay gothy guys make out with each other, and sexual exploration with her would more focus on peeping, getting peeped on, and her engineering situations where she puts on a show for you. The overlap with both characters may have is that both may be somewhat nerdy about their sexuality, i.e. reading educational books about sex and being somewhat more informed about outlandish practices than the average girl of their age.
The point is, pick either. Or make it multiple, different sisters if you're focussing on making it a sister-fucking simulator. But at the same time, you can just make them not related to you. Having the sister be the only character that's hard to get, has a story and everyone else is a turbo slut where you take em out on two dates to get the horniness points up and plow them, is kind of a trapping of the genre, I think.
I'm also a bit worried about
but then maybe you've blown your chances with the sister.
Or the (gothy) sister engineers your sexual encounter with the girl because she wants to fuck you by proxy, watch / listen / peep as you plow the other girl, where the punky girl might get genuinely jealous, figuring "a slut like that" isn't good enough for you and figuring if you want to lose your V-card that badly, you might as well do it with her, and find a quality girlfriend when you're more experienced - Or so she thinks, because the more she is playing the sexual guardian, the more habitual the jealousy and the sexual encounters become, the more she treats you as her only real boyfriend because her other sexual encounters never had as much emotional impact, and you begin guarding her as well like no one else has ever done.
be very hard to get, very demanding and cynical at times
Girls aren't really demanding, what they want most of all is attention, get touched and getting fucked. A femdom-type sister may well be an entirely different, third character again, one that has a serious sadist streak and gets off on manipulating you into doing embarassing things or utterly submit to her beauty, unable to control yourself or maintain dignity in the face of her sheer hotness. Though I think the archetype is overdone and I've never seen it done very well, but then it's simply not my fetish. If that sort of thing appeals to you and you think you can write a compelling character like that, you can try working it into the gothy sister, as her "no touch" sexuality and vanity are more compatible with that, but it is utterly incompatible with the punky, permissive type. But in the case of a very demanding girl, it may well be better to make her something not related to you. Fucking a hard to get girl, femdom type girl that is also your sister will just end up in endless contrivance that gets people out of the "zone", while writing a story about fucking a permissive, lovable hippie sister (with the MC being much more conflicted and shy about it than the sister herself) should be easily doable.
Make it a love triangle between a hopelessly horny but inexperienced MC with a gothy neighbor girl with a fucked up sexuality (initial point of contact may be peeping on each other through the window) and a permissive hippie/punky sister that jealously tries to dissuade you from going after the neighbor girl, seeing as how she leads guys on and on and manipulates them like an evil bitch, and bam, you got the roots for a compelling story where each character has a motivation, and where you can weave in drama and humor and titillation.
