How do you change your maid of honor?

im getting married next year and i had initially asked my sister to be my maid of honor, but i cannot have her do it. i am going overseas so most of my wedding planning will be in the hands of my fiance, parents, and the bridal party. my sister is not reliable and already refuses to talk to me and i have not been gone for 2 days. she never congratulated me on my engagement, and although it was agreed previously that we would be each others maid of honor, i just cannot have her. what is the best thing for me to do? i have tried to get along with her and i thought that asking her would help us get closer but it didnt.