I agree. Incestuous Casting should remain a Trivia trope opposite Cast Incest, while a new trope should cover the "siblings pretending to be lovers" thing.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"If it's for a Show Within a Show, that's just an in-universe example of the existing trope, and it shouldn't need a hard split.
The logic only applies if we treat Show Within a Show very liberally. Let's say Alice and Bob are siblings. They get involved in a Zany Scheme which requires them (either by design or improvisation) to pose as lovers. All of this is taking place in-story, but it's not Show Within a Show. And it's an actual trope, unlike Incestuous Casting which is strictly trivia.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
Well, if it's a Zany Scheme where they're pretending to be lovers, it's Square Peg Round Trope because there isn't any casting going on. But if it's a school play or something, it's definitely this trope.
Correct, and thus an In-Universe example. That's completely appropriate.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"Very often people who make fiction will put things about themselves into works. The trivia that we find about them, they are likely to note as well. Doesn't stop it from being trivia, it is like the inverse of the Real Life section, it's just interesting to note that they recognise it too and the fact that it exists in their works demonstrates that it exists and is common enough to filter through.

Currently Incestuous Casting is a bit muddled in the examples category with two main ideas there.
One is a Trivia item where the two people playing lovers on a show are really siblings.
The second is an in-universe trope where two siblings pretend that they're lovers for a play or other deception.
I propose that we split the first from the second.
Reality is that, which when you stop believing in it, doesn't go away. -Philip K. Dick