On one hand, it's must be SUPER weird to date your adopted brother. On the other, it's very much Richie/Margot in “The Royal Tenenbaums.”
The good people at Crushable seem to think their relationship is pretty much confirmed:
Although she never officially adopted him, the 22-year-old Gordon lived with Whitney from the time he was 12 years old, making him more or less an older brother figure to the 19-year-old. A few days ago, he tweeted the above photo of the two of them, which some people believed to have amorous overtones. Then yesterday, he referred to her in the possessive, which also seemed potentially…something ... and earlier today, he all but confirmed it."