They are too regularly familial. Close enough that there's no weird tension between them, but not overly close.
Incest pairings typically arise from one of these two. Either the characters have (for whatever reason) a distance between them that alters their normal familial dynamic (e.g. Elsanna, where they are effectively separated as young children, creating this strange longing not typical to a pair of sisters; or Ryuuko x Satsuki, which like Elsanna features separation and then tension resolved into a desire to have a family)--OR the characters are unusually close in such a way that they seem to carry a special significance. The most common place this side shows up is in instances with twins in media, because they are almost always depicted as being basically two people that are one, who go everywhere and do everything together to the point that it almost seems odd for them to be interested in anyone else.
Enabler isn't either of these. They are two really normal sisters with the really normal amount of distance who fall perfectly into the big sister little sister and good sibling friends dynamics. There's no tension or overfondness or anything to extrapolate the incest from, so it becomes a case of shipping just for the sake of shipping.
I actually really hate most Enabler stuff, it seems like arbitrary incest just to fill out the RWBY pairings, and most instances of it seem to be extremely forced. >>2480291
I think if it were going to be done, you'd need to just abandon Yang's actual characterization for how she interacts with Ruby and instead make her that typical incest character, who despises themselves for their thoughts and urges but increasingly can't help but see the other person in a sexual way.