NTJ and you should still use the name. What harm is there in two people having the same name? And the best part is, when they grow up and ask why they have the same name, you can tell them how stupid and shifty your brother and SIL are for stealing a name from you. And I'm a HunterxHunter fan myself and plan on naming my future son Killuah so I feel you on that. Lumi is a gorgeous name and they're petty AWFUL people for doing that. That's so scummy of them. There are literally MILLIONS of madness to choose from. They stole your idea and hoped to force you to change it. Don't let them do it. Stick to your baby name and let them cry about it later. That's their own jerk fault.
NtJ and my advice is to keep going and just LOVE ALL OVER your husband the whole time. Completely rub it in her face. Be OBNOXIOUS about how wonderful he is and how you're so happy your lives have lead you two to each other. Make her so uncomfortable.