Well, you'd have to go around the world to ask that one, but actually, you can't. Unless someone tells you, "Congratulations! You are allowed to legally disown your sibling!!!" That would be a miracle.
you yourself can not disown a sibling because it is up to the parents on judgement, but if the older sibling was to become the legal guardian than yes you could disown your sibling.
yes im courently in love with my step sister me and her hit it off when we played barbie when we where 6 and have enjoed the pleasures of ben lesbian ever since