This thread's been pretty active lately it seems.
Spoiler for Nise 2:
Regarding Tsukihi's screen time in the novel, you have to remember that the main character is never any of the girls. It's always about Koyomi and his relationship with each girl. Tsukuhi Phoenix was about him coming to terms with the fact Tsukihi was never really his sister in a sense. Yet, in the way that it actually matters, she was and will always be his sister.
P.S. Considering that Nishio Ishin spent part of the novel explaining Hitagi's progression it's pretty obvious that he already expected reactions like stuopidget's
.
With that said though, I still find Nise 2 to be the weakest of the novels.