Her marriage is void without your consent and she can’t marry a non Muslim and she can’t be engaged to one! By doing this she is either a kafir or a fornicator! Your mother is in great danger as well for approving this.

I am the wali of my older sister and she is engaged. I never approved of this. My mother told me one day she was engaged. He is not Muslim but she says he wants to become Muslim. He lives with her and she doesn’t want me to be included in the wedding and my mother tells me to stay away from my sisters engagement. Would I be justified if I don’t attend the wedding? What should I do?