I'm not sure I understand the dynamic here. I can understand the mangaka not wanting to be exposed, but why would the stepsister not be like, "Yo, love your work!"?
I think because it's embarrassing to tell your step-sister that you love her romantic step-sister comics.
I'm reading her immediate reaction as like half fear and half utter excitement, expectancy, and hope of being ravaged. It's like, what are the odds that a prominent writer of primarily step-sister yuri is coincidentally heavily into step-sister yuri? I'd personally say pretty good odds. Goes the other way, too. It ain't a coincidence that she recognized her so quickly.
In any case, sign me the fuck up. This is bound to be an amusing one, regardless of what flavor of the story will be.