Unsure of what to say

I wanted to ask on here for maybe another perspective. My niece is 10 years old and today she asked me for the second time how is Sam Smith gay? (she heard it from school) Her mother (my sister) is a practicing Christian and doesn't believe that someone is born gay. I'm not not out to her or any family so she's never explained the concept of someone being gay. Do I respect my sisters parental boundaries and tell her to ask my sister? Or I do tell her that some people like the same sex but are no different from mom and dad. The only thing I worry about my sister telling her is that she'll tell her it's wrong. My sister doesn't hate gay people and would never tell her to hate them I just don't think she'll stress the ideas of tolerance and equality. So I guess my question is do I tell her? What do I say? And is it overstepping boundaries to tell her?