Ok, for real though, why does Nokki keep hanging out with her?
Stockholm Syndrome.
More seriously, Nana is pushy, unpredictable, and calculating, but she lacks genuine malice. She took a shine to Nokki and even though she finds teasing Nokki funny, Nana has always treated her with affection. Even years later, Nokki is a welcome guest at the Aizawa household, just as loved by Mana as Rie and Shiori.
Heck, given they graduated in March, that means the earliest this happened was when Nana was four or five months pregnant.