hot take but like bianca di angelo was actually picking the best choice for her and nico. imagine being TWELVE and taking care of your brother for most of your life. not only are you literally from another period of time and quite literally still trying to understand this new world, you believe that you are orphaned with no one else in the world who cares about you.
bianca was twelve when we meet her, and she is quite literally raising nico on her own. she was adulting at age twelve. she had to manage their budgeting, feed nico, clothe nico, and overall take care of both of them. she was a child too, and I know so many people hate her for abandoning nico, but at that point I don't blame her for feeling like her only job was to take care of nico. bianca would always care for nico, but who would care for her?
and then she meets percy, who not only promises to take care of them, but seems to make nico happy. bianca could get nico all the mythomagic cards and statues, but it's nothing compared to camp half-blood, his dream come to life. she is promised they will be taken care of, that for the first time she can remember, they will not be abandoned.
and for the first time ever in her life, bianca gets to join a group of girls just like her and actually make friends. for the first time, she finds a group that promises that she will feel loved, that she will be taken care of.
ultimately, i don't think we should be blaming bianca for her decision, because like everything else in the pjo world, it was the gods' bad parenting that led to this.