I WANT TO MARRY HER.

I want to marry my big sister,I act kinda distant with her because I try to avoid feeling things but I can't keep it in. I really want to marry my big sister she has taken care of me since childhood we are just 4yrs apart and I feel scared of confessing my feelings to her. Yeah I know this is taboo,she won't accept it but atleast I should try right what's the worse it could happen. I'll lose my big sis, I'll Probably called crazy but if I don't say this then I'll live my life regretting it. Am 18 next week and I plan on confessing my feelings that day after writing what I should say. We are very close and I really love her, give me some advice.