I don't think incest is unethical anyways. It's sure as hell not for me, but it doesn't hurt anyone, and it's none of my business anyways.
If said sister was not related by blood, and I hadn't known them long enough for us to have anything resembling a sibling bond, then I wouldn't have a problem with it.
It isn't really incest, there's nothing really there that resembles a family tie.
As for the parents on the other hand... That would be awkward at best and a disaster at worst, depending on who they were. I probably wouldn't say anything to them out of curtsey, can't even imagine how uncomfortable that would make them.
So I say do it, but don't let your folks find out, at least not until you can get your own place.