Not because of Shinobu. It's because Oshino was gone, he had to start making decisions and dealing with stuff on his own, and for that reason Oshino thought Shinobu everything he knew. He knew that at some point he will once again try to help someone. He had to deal with everything personally so he had a lot more screen time and development.
It was actually quite natural but still it was his sister spotlight as it based primarily on dealing with his sisters problems without telling what he is and keeping everything a secret from them.
How Karen developed was brilliant, but still I think the most surprising events were around Tsukihi, even though it was very short, her arc was incredibly well done. And how it all got finished was also superb, even though it didn't show much, what was important is the bond between siblings.
The most important part of that bond was when Araragi asked if Karen-chan would die for him, and she said yes, and if she would die for Tsukihi, she said yes, both without any hesitation, no pause at all, no thought process needed. THAT was incredible.
