Well it's not all that clear that it is offensive in any way and it's not all that clear that it isn't a name that is used with her very commonly.
From her USA basketball site in her personal notes section....
Personal Notes
- Born on Aug. 27, 1994 in Syracuse, N.Y., and goes by the nicknames of “Bre,” “6-10” and “Stewie.”
- Daughter of Heather and Brian Stewart; has one brother, Conor.
- Aunt Casey Villarreal is in the U.S. Army Reserves.
- Is a scholar athlete and member of her school's honor roll, as well as the recipient of the Outstanding Character Award.
- Has volunteered locally on behalf of the Ronald McDonald House, the March of Dimes, the North Syracuse Optimist Girls Basketball Program and a youth literacy-outreach program.
- Started playing basketball in the second or third grade.
- Tweets under the handle: bre_stewart30