You mean Jill's friend not sister. Bella is only her friend. Jill's sister is dead. And it wasn't meant to be comical, it was meant to be an insight on Bella.
About the internal monologue, i disagree, i find it a nice feature. In general i like them. As a note, they dont need to be necesary, most of the time they are not. But that doesnt mean they should be removed at all. In a book you dont need a description of the place where the action happens, yet they make the book better.
In particular I specially like Jill's inner thoughts. For two reasons. First, this one was meant to be comical. It's a clasic joke, where we seen a character struggling with himself by doing the oposite of what he/she wants. Not the best joke of the game, butnot bad neither.
Second, since it's a porn game, and Jill is a platonic relationship, thats her charm. We dont need Sage inner thoughts couse we have her pussy (the "real" thing). With Jill (for thoose who like her) we get a different thing. Platonic relationships work with fantasy. They are two people fantasising with each other. It happens only in the mind.
After all Jill's inner thought's included something like this:
View attachment 1463122
Now, i am not fan of platonic porn. But there are a lot of Jill's fans. I bet at some degree they like that. And if i was a Jill fan that would probably my favorite scene. And i dont meant couse that's the first time we actually see her pussy.