What is the ruling on a niece invoking evil upon her aunt, and will God answer her prayer?

You did well by honoring your guests, as this is something encouraged by Islamic law (Sharia), and it is the Sunnah of Ibrahim (Abraham), peace be upon him. It also preserves one's honor and increases affection and ties of kinship. Your niece's prayer against you will not affect you as long as you did not wrong her, for a prayer made without right is not answered. You should meet her ill-treatment with patience and kindness, and strive to reconcile her with her mother-in-law.