Why can't my sister accept who she is?

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I am half Filipino and half white (with some Amerian Indian mixed in. I am proud of my heritage. My sister on the other hand is another stor. She is full Filipino but insists on telling people she is white. When she fills out forms asking about race she marks the Caucasian box. When people ask her about her ethnicity she says she is white. She even told me that I should not mark down other or mixed on questions that ask about race because Filipino is white. I know that it is her life but it upsets me because it is like she is trying to deny her heritage and it upsets my mother who feels insulted by my sisters actions. I just had to get that off my chest. Has anyone had any similar situations with family members or friends?