What is the ruling on shunning relatives if one of them slanders and fabricates without proof, and is it permissible for a father to shun them in defense of his daughter?

If the aunt's accusations of injustice and slander are true, then she faces a grave danger unless she repents, for Allah Almighty says: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne a slander and a manifest sin." She should be advised and reminded of Allah, and prayers should be made for her guidance. If she does not respond, it is permissible to shun her to an extent that stops her harm without a complete severance, because she is a relative with whom ties of kinship must be maintained. And if she commands her daughters to cut ties with you, then this is a reprehensible act in which she must not be obeyed.