How is being a porn actress "out of context"? It's not like everyone are well adjusted members of society in this game. The story is about fucked up people that does fucked up things. Being a porn actress seem perfectly normal in that context. Rather, it is pretty tame compared with the shit MC has been up to.
I mean the context to why the dev changed her into a porn star when it wasn't originally written that way. If her character was originally written as a porn star, I would accept that's how the dev wants her character to be. For example, Carlie from
Depraved Awakening is introduced as a prostitute, she wasn't changed to be a prostitute later. So I have no problem with that. But I have no idea why the sister in this in now a porn star. Is her being a porn star now important to the story, but not before? In what way is her being a porn star now relevant to her character, but not before?
You're right that people do fucked up things. So why a porn star? Why not write her as a drug cartel assassin then? Just because the world is shitty doesn't mean characters can just be anything without reasons (at least to me). Like, if a character murdered someone, wouldn't you want to know the reason why, or would you just think, well people do fucked up things, who cares if there's a reason? That's just how the world is!
Also, if her being a porn star is no biggie, why does the MC get upset when he first hears about it? If it's "normal" he should be fine with it then. Why not have the MC be cool with her being a porn star, if we the player ought to be cool with it then? Maybe have MC be interested in doing porn too then? Have the two do porn together? Why not, this is a "porn game" after all. Have MC's co-worker also do porn. See, now the sister being a porn star would make sense to me, since she gets everyone in the story do do porn.