this?

No Mr Bond I expect you to die

In vol 5, the black cat kissed the brother, "putting curse" on him before he left for USA to take back his sister. It is final proof that she does have strong feelings for him, making her a formidable rival to childhood friend and the sister. Before this also, she invites him to lie beside her on his bed while writing a visual novel.

In vol 7, the black cat who become white cat (literally) decided to grab the brother away from sister and make him her lover in the end.  She wanted to punish the sister for not being honest with her feelings but also she developed feelings for him (during the story arc when the brother is helping her making a visual novel, it is quite clear). That's why you seeing a lot of drawing depicting her in white dress and summer hat, she is now in very soft, feminine mode. (hence the nickname Shironeko)

Of course, the sister is not amused. The happy otaku girls trio now in danger of splintering with 2 girls are very angry with each other while the glass Oujo Sama is bewildered on what is going on. To her, this is last thing she wanted, loss of tight group of friends like previous group.

This vol has a lot of drama, beginning with brother going to "date" with his sister which finally spotted by their mother.  Which led to an angry confrontation between mother and son. While on the "date", the sister was gloating haughtily when they ran into childhood friend and black cat which she acknowledges both of them as rivals for her brother's affections.  The black cat literally went BSOD when she saw them together, the sister was smirking at her reaction as if she won a victory.

When she is sort of "rejected" by her brother, the sister got emo and date another guy to piss him (and her father too  ) off. Which turned out to be a fake bf after very intense confrontation in the house between 3 of them. Lots of cake were thrown around, literally.

I don't think anime will reach here but I did not finish the series anyway (I drop it at ep 4). It is strange to read this vol while the author asserting that tone is still comedy, given the amount of angst and drama here.