Why is this happening right before the start of the last volume
Because it's the climax. This is my guess: Fiancée will show up, go "stay away from her!", MC will go like "oh no, she had a fiancé, now I can't be with her!!", and everyone will be sad, right before either MC goes like "no, I can't accept that!!" and stands up to the fiancé and the brother despite them being super rich and powerful, and they'll go like "this guy's got guts, and he also truly loves her, guess we have to accept him", and then they get together, the end.
If I turn out to be right, sorry for the spoiler. EDIT: I was not.