Bobbi Kristina Brown -- I'm Doing Oprah Because I TRUST Her

Bobbi Kristina isn't doing her first post-Whitney interview with Oprah Winfrey because of the money -- she's doing it because the big O's a close family friend ... and Bobbi trusts her to be fair and respectful.

Bobbi is telling friends -- Oprah "was loyal to my mom, and never did my mother wrong, or made her look bad. She always looked out for my mom."

As a result, we're told Bobbi feels Oprah would show her the same respect -- not bringing up issues that could potentially show Bobbi in a negative light ... i.e. substance abuse.

According to sources close to Bobbi, Oprah was one of the first people who reached out to her after Whitney passed.

So far, it's unclear if Bobbi is getting paid for the interview -- which airs this Sunday.