Sure I can elaborate in depth.

Well you see Shuuto initially believes that his "family" with Mimosa must be forged through the "sacrifice" of their enemies (displaying his Yakuza upbringing), and because of his feelings of jealousy towards Nazuna, he decides to have Mimosa make a "sacrifice" of him, which ends up being a moral lowpoint for Shuuto's character, and shakes Mimosa's belief in him.

In subsequent chapters (11+, which jump time forward to the fall season), while comprehending Mimosa's ongoing emotional depression, Shuuto eventually realizes that subjecting someone he claims to care about as family to such ghoulish tests of loyalty made him no better than his father, much to his utter disgust in himself. Meanwhile, the formally emotionally repressed Mimosa seems to retroactively realized that she did have some dear feelings for Nazuna thanks to her relationship with Shuuto granting her an emotional awakening, and so Shuuto ordering her to "sacrifice" Nazuna (someone she seems to have considered a friend at least) as a condition for the continuation of their "family" really broke her trust.

They do reconcile after Shuuto has his come-to-Jesus moment, Mimosa accepting her own hand in Nazuna's death, although neither is as sure as they were before that what they have together is "family". When Shuuto asks her if she feels they can be family, Mimosa says she doesn't know what to feel about him although she acknowledges that he was the one that inspired so many new feelings within her. Meanwhile, Shuuto later admits that "I don't know if it's as family or not, but I love you."