The protagonist being Haruki's Onii-chan is likely unrelated. It's a patchwork family with Haruki and her mother making up one side and the protagonist and his dad being the other half. As things are now, they are not blood related. Still, Haruki's mom did state that the protagonist, unlike normal humans, has some kind of resistance, being his dad's son. So there is likely a hereditary reason for that matter as well.
What do we know:
Succubi can change their body to match the person they like/love. Haruki became a female and his feelings or desires were quite important for that to happen. Kanata fulfills this condition, even if only subconsciously. At this point it's still unclear if having a succubus ancestor is a pre-requisite for being able to change genders. It's likely so, but not confirmed.
It can be assumed that coming in contact with a succubus' Saliva is normally not sufficient either - Haruki also shared water she drank from with another male classmate and that one did not become a girl. He got aroused instead, which is what you'd expect to happen for normal humans:
A succubus' saliva contains significant power - likely all body fluids do, e.g Sweat. Haruki emits lewd scents depending on her desires, i.e. she releases pheromones that can attract/affect humans. It's also likely that the scent can make other people accept Haruki as a girl without grossing the people around her out. And her saliva also holds powers. Mom confirmed both, stating her saliva acts like a strong aphrodisiac and induces lewd dreams. That said, even with how playful and mischievious Haruki's mother is, she wouldn't have left out something as important as a potential genderswap effect that might happen to someone who ingests Haruki's saliva.
So I would draw two possible conclusions from this:
1. Kanata is also from a succubus bloodline and Haruki's saliva likely triggered Kana's dormant powers
2. Haruki's power can affect other people if they're somewhat close to her, ingest a body fluid and want to change genders
It's likely the former...