Briefly, my main character is a young woman who finds out that she's adopted. Which would be stronger? For her to think she's white/black and adopted by a black family and then find out she's white/Indian? Or for her to be prejudiced against Indians and then find out she's half-Indian?
The latter. Either way, she's finding out that she's something she's not, but in finding out that she's some part of the race she's most prejudiced against, it would screw with her head more. She'd have come to terms with her own heritage.
I think she doesn't know she is Indian--You mean Native American or actual Indian from India? Either way if she/he hate that which they are makes a more powerful story in my humble opinion.
There is nothing simple about prejudice. How much any particular factor affects a character is one of the many choices you have to make, and to fid a way of showing. What does being black mean to her? She could be so light skinned that no one other than a racial purist would even think of her as black, but she could feel that a single ancestor of color makes her feel tainted. If she is Indian, does it matter exactly which ethnic group(s) from India is in her heritage? Or is she more upset that her parentage has been concealed from her? It is all about perceptions, and therewfore is also relative to who is perceiving. Were her adoptive parents ashamed of their child's true parentage, or perhaps smugly proud of how liberal they were? Was it a point of contention, or of silent resentment, between them. So many choices! But isn't that part of what makes writing so exciting?
I agree completely here... Had a good friend in school that was as pale as pale could be. It was quite surprising to see her biological father, who was very dark skinned-black. Personally, I always loved that contrast. One issue however, with the OP. Just because someone finds out that they are a part of something they don't like, isn't a reason for them to change their mind. If anything, it can cut the exact opposite way. She can rationalize and/or reinforce the racial bias and stereotypes with the information as well. Something along the lines of, "See, I was right all along. My own biological mother had to give me to a white person because she couldn't raise me." I'd make sure that I had a different reason other than just being genetically related for the changes to happen. The human mind is way too adept and rationalizing for that to be the only thing that pushes a transformation.
She should be prejudice to whatever race she finds out she is, but her adoptive parents shouldn't know what race she is either. In my opinion the adoptive parents should be the ones who made prejudiced against the race; that way she feels betrayed and has self hatred at the same time. This is just my opinion though, you should do what you think is best.