Is there any way I can save my sister from a FWB situation?

Note: If I have violated any guidelines or done anything wrong in this post, please politely let me know, since this is my first time ever posting on a forum like this.

Hello, FDS queens! I'm an 18F, curious if I can save my 22F sister from putting herself in a bad situation. My sister (let's call her Jane Doe) went out on a date with a guy (we'll call him John Smith) once back in August, and has been teetering on the edge of disaster since. Every time my sister and I are by ourselves, she brings him up in conservation -- but it's never anything romantic. Right now it seems that Jane is more on the "friendly" side of FWB with John, but I can definitely see things become more serious between the two of them when this semester ends and they both have less responsibility to worry about over the summer.

Here's a horror story in the making: Jane and John are both involved with the marching band program at their respective college (which I will not name drop for privacy's sake), and, as such, both have ended up on the same football-related trip with each other several times. Well, one time, it was late at night and Jane had been talking to John behind her friends' backs (since they all knew he's bad for her), and she ended up inviting him over to her hotel room late that night so that they could finish their earlier conversation that had gotten interrupted. As far as I know, nothing sexual happened between the two of them (they just kissed some), but when John told my sister that it was getting late and he didn't think he'd be able to get back into his hotel room since his roommate was probably asleep, my sister let him spend the night on the floor in her hotel room (an obvious sign to me that my sister lacks strong boundaries, especially around guys she's into).

But, if that wasn't bad enough, a couple of weeks later, the two of them, and some other friends, were all drunk in a hotel for another band trip, and while Jane was drunk, she tried to convince John to walk her back to her hotel room so that she could go to bed, which he obstinately refused to do, so she ended up humiliating herself trying to get him to leave the room with her in front of all of their mutual friends while inebriated. Once they got back to campus and all of the band students went their separate ways, Jane apologized profusely over text to John for her "misconduct" (if you could even call it that). And what did John do in response? Literally left her on read, and said nothing in return.

Smh, and if that didn't paint a clear enough picture of how much John is using my sister, this is literally how Jane and John had their first kiss together: it was the night after their first date, and the two of them were hanging out at a party, when John kissed Jane, and the two ended up making out at said aforementioned party. And then, of course, the best part: after making out with Jane, John then tells her that he isn't "looking for anything serious" and that he doesn't want to be in a relationship at the moment.

That was in August, and this "romantic entanglement" between the two of them is still dragging on to this day. I even recently tried asking Jane what she thought John wanted with her, and she told me that he was probably just "messing around". Which frustrates me even more because my sister knows that this degenerate LVM is using her, yet she keeps interacting with him anyways. And she won't listen to anything that I say about it because I'm just her "annoying little sister" (not even joking when I say that she rolled her eyes at me when I brought up a concern that she might regret giving her body to this LVM if things ever become sexual). What's even worse is that Jane admitted to me that she brought John over to the house she shares with her friends when the house was empty because she knew that one of her roommates disapproved of him.

My best guess as an armchair psychologist as to why my sister keeps interacting with John is because she's only been in one true relationship before and it was with a LVM several years older than her that spent most of high school grooming her to be his backup for after he dumps his gf. My sister is aware of the fact that this "relationship" was actually just her being groomed for the role btw. My thought is that she keeps talking to John (even after he ignores her for several days and then apologizes over text because he's "been busy", which he did just last week) because she genuinely wants to be in love, but since she has low self-esteem and is a libfem, she is okay with settling for a FWB because that's better than hooking up to her, or being single with no one "pursuing/interested in" her (*sigh*).

Anyways, that's as much as I can explain without writing a full novella about it. Apologies for the long post. FDS saved me from what would have ended up being an abusive relationship with a male narcissist last year, so I'm hoping that some of y'all lovely ladies can give me some advice, or maybe some resources, that I could send her way that would help her snap out of her trance.

TL;DR: My sister keeps interacting with a guy that is clearing using her, she's aware that he's using her, yet she keeps interacting with him because of low self-esteem. I want to know if there is anything I can say or do to her to save her from what could end up being a really awful situationship fate.