Akatsuki got an emotional dependency issue. She wants to be close to Yume and become her family, so she came to a conclusion : marry Mizuto ==> become Yume's sister.
Later, Kawanami who is Akatsuki's childhood friend and neighbour, will tell a horror story about her
Akatsuki had a boyfriend back in middle school. Few months after they start dating, the boyfriend freaks out and break up with her because she spoils him so much he felt like he is not allowed to do anything. The stress gave him gastric ulcer and he was hospitalized. After that, the guy become allergic to female affection. After losing her boyfriend, she look for another person to attach to and found Yume.
if it's not obvious enough already, Kawanami was that unfortunate ex-boyfriend, that's why he knew how to handle her. Akatsuki even went far enough to install tapping device and buy tool to listen to voice through the wall (their bedroom is separated by just a single wall). Even before that, they've been staying at each other's house for years because both their parents are working and there's no other kids their age in the apartment they live in.
The development won't be as dark as how the description made it though. Akatsuki's story is in volume 3.
About Mizuto
We could say he is still traumatized by how his relationship with Yume failed the first time around.
He holds back from trying to rebuild a romantic relationship and wants to have enough by just being siblings like now.