I would consider it Backstreet Boys fanfic if the fact that the original character was related to a backstreet boy was important. If say, Howie was just their parent and Howie could just as easily be replaced with an average joe with minimal impact to the overall plot then I would say it's not BSB fiction. However, if the story was about Howie's son dealing with the fact that Howie was a backstreet boy and kids sang I Want It That Way all the time to tease him, I'd be more likely to classify that as BSB fiction.