if incest fantasies are unacceptable than every other slightly problematic fantasy gets the boot as well
the reason its acceptable is because in fiction its fuckin harmless, generally speaking
people deserve to have outlets for this stuff. even the stuff you think is gross. plus incest fantasies dont even equate to wanting to do anything with actual siblings.
like as much as media can influence culture and society, i dont really think stories about sisters crushing on eachother that are honest about the fact that they're indulging in a romantic or sexual fantasy are that harmful. hell stuff like that can even be helpful for victims of abuse.