The only thing crossing my Mind when i read the OP was not that his/her Sister bases it on Racism, but preference.
Its really simple, the whole Interracial Angle is very alluring to some People, you know, Asian+White, or Asian+Black or what have you. In short, the OP's Sister likes the Idea of Interracial Sex, simple as that. Dont really see how thats in any form actually racist, although the reasoning is utterly stupid, given that anyone can cheat on someone, even if you dont, the option, that chance is always present, unless of course you are living alone in a cave somewhere as a Hermit.
Or in very short terms: She (the OP's Sister) personally prefers Black Meat over White Meat..oh god i cant believe i made that analogy. But you get the Idea.