Romantic ship aside, I really enjoy the chemistry between Yamada sibling and Yuu-Mahi (fake) twin. Kaoru is a shameless, overconfident womanizer when she's outside but when she's at home, she pushes Honoka in any way possible, takes any chance to bond with her (aggressively trying to get in the bath with Honoka), happily dressed Honoka up when she asked for her help. She always shows unfiltered, uncalculated admiration towards Honoka. Honoka, on the other hand, while looks undeniably annoyed with Kaoru's antics most of the time, she still trusts her sister a lot, looks out for her and even goes along with her sometimes.
At the start of the manga, Yuu disliked Mahi twin a lot (because they were really loud and kinda weirdly childish, she couldn't click with them). However, as time passed, they naturally grew on her and started to consider her their big sister/mother figure. Yuu let them cling onto her as well. It was unusual for Yuu since she's a grumpy cat, ice cold queen type of person. Hella adorable.